No idea what you're trying to say...
You do know how to search the forums right?
If not lemme know, & I may dig them up for you.

No sir, I have no idea.

Compared to the N900, the N9 is better to take calls, better in bright sunlight, better in out of the box battery life, better in demonstrating how fluent Maemo could have worked with just that little bit more of tweaking and 1GB of RAM...
That's about it.
Did I miss something?

I suspect that OP knew the answer to the question before he even asked it, since the answer is quite obvious. The N9 is not a suitable replacement for the N900, except in those cases where the user does not require anything other than a typical smartphone. This also applies to the N950 IMO, since they run the same software, and that software is not designed to be used with a HWKB, which limits the advantage of having one. Also, I find the HWKB on the N950 to be cumbersome for handheld typing. It's too spacious for its own good, which means that your thumbs need to travel much further for each keypress. It does however make it easier to use on a table/desk. In all honesty, I prefer the keyboard on the N900.

N900 + more RAM + fast CPU = WIN!

I did not know the answer. I wanted you fine folks to give me your direct opinions on the matter. The hardware keyboard is a big plus for me.. and Aria (second post) basically answered the question elegantly for me.

I believe the N9 will perform better in its version of n900fly, since its shape produces less drag.

That's pretty much it for functionality, I guess. You'll probably be better off with the N900 for all other purposes.

As I've said almost everywhere, the only drawback is the camera on the N9, otherwise I am more than satisfied with it

You're not really planning to buy an N9 are you? I get the feeling you were bored at work or something...

Sigh, these threads must die....
They serve no good purpose but to get N900 & N9 users to fight against one another.
And a time when we need to be supportive of one another more than ever before.

Except that doesn't exist.

There are many regression sw-wise, especially at the upper-most layers, we know that.
But we're finding with each month that work-around/twks/mods nevertheless are appearing.
People were originally doomsdaying, saying it's a lost cause, lets see what things look like in 6mth.

The "biggy" is a fully functioning & unfettered AEGIS open-mode.
I haven't checked in the last 48hrs, but when I did it was still somewhat bogged down.
It comes in "fits and bursts", but I think we'll get there in-the-end.
Software is where the biggest disappointments are, but sw is a very rubbery thing.
What we think may always be a lost cause, can be substantially improved over time.

HW-wise the N900's certainly not superior...
Yes when the N900 was 1st released it was far more aggressive/bold hw-wise.
The N9 should've been way more of a bump, but overall a bump it still is.
Here are the N900's notable hw advantages:

mSDxx
No SD2.0 support means >32GB support will be possible, but it'll never perform as mSDXC should.
N9's built-in 64GB eMMC will perform as well RW-wise as the N900's 64GB mSDH/XC.
If you use N900's two mediums simultaneously that may change, or if you go above 64GB mSDXC.
But the latter is still a pricey proposition....

IR
Up to you, I personally never used it on the N900

HW QWERTY
Ditto here, I've always used BT/USB portable qwerty's.
Far better for serious typing than built-ins.
(but the N900's qwerty was the best built-in I've ever used)

FM-TX/RX(RDS)
RX(RDS) is almost done for N9, & TX (unlike N950) isn't ruled-out yet

Speakers
The speakers can get louder with less reverberation issues.
The actual sound quality is not better though.

No doubt I'm forgetting a few other things, but that's mostly it.
The list of advantages the N9 has is somewhat larger.
I'll leave that for you to do some research on.
But if the above features are critical for you, then the N900 is what you want.

i just buy a N9 because my n900 get damage ( just the radio part ) so i still can play with my n900 but not take call anyway the N900 is another kind of beast, one that i think will never be another one again , for a device from 2009 and counting all advance in the area is pretty impressive what n900 can still do know but still is a old device and that is why N9 good for follow the maemo device legacy in this time just need wait what community can make with the N9 this time i pretty sure a lot of amazing things will come.

I agree with the last point, but not the first two. I use and carry both an N9 and an N900 (which I agree is slightly ridiculous...) since the N9 can't replace the N900 for a variety of reasons, but there are some things the N9 does much better, for example browsing mobile sites, reading and even writing email (SWYPE is great), instant messaging, usability when it comes to various kinds of feeds, and Nokia maps, especially outdoors in daylight.

For regular calls they are neck and neck, but for SIP calls I have found the N900 to be better, and SIP is what I use most of the time. I have had issues with SIP on the N9, basically calls breaking up or dropping altogether. Trying the same number with the same reception (either wifi or 3G) on the N900 has always produced better results. That's just my experience though; your mileage will vary.

Better out of the box battery life is true for the N9 I guess, but the point is moot because you can carry as many spare batteries for the N900 as you want with you and they are dirt cheap. I currently own 8, but generally carry 2-3 spares with me in my wallet when I'm out and about, where I don't even notice them. I always charge the batteries, never the phone, so I never worry about being attached by a cable with the N900 (and I do all my file transfers with ssh over wireless). I always have at least 1 spare battery to reach for when I need it, wherever I am. Basically I never even have to THINK about battery life with the N900.

The N9 is very slick and nice to hold in the hand with its unibody design, but the lack of removable battery is a huge pain, and at the end of the day the slickness and coolness of the phone's body just isn't worth this extra inconvenience. I find that the battery life varies hugely depending on how much I use the phone. I can easily get through two days with very light use; but with even moderate to heavy use suddenly I can't even get through the day; by tea-time the battery is finished. So I WORRY about the battery all the time with the N9, a huge downside to ownership and every night I have to remember to charge it. Having to worry about your device does not make for happy ownership. In contrast, I never have to worry about how much battery life I have with the N900, and I never have to remember to charge it. Whenever I wander past one of my chargers (there's one in the kitchen, another in the bedroom), with its prominent blue/red LED, seeing the blue light (means charged) reminds me to swap the nicely charged battery for the empty one I'm carrying in my wallet, which I do right away, and then forget about it again. It's a system that works really well.

If it came to an actual choice between the N9 and the N900 though I would find that very difficult. They have different strengths and weaknesses. I guess I would choose the N900 over the N9 at the moment, with a heavy heart, but this is only because there is much more software available for the N900 and it's such a mature device. There are things I simply can't do on the N9, but hope to be able to do as time goes on and more software is released. Eventually I expect that the choice will then go the other way - again with a heavy heart.

I guess I feel lucky I don't have to make this choice.