Sure, but what is going to feel the most comfortable and natural?

Using the arrow keys on a PC keyboard is with your fingers, D-Pad on a gamepad is with your thumb.

Since this is likely to be used with your thumb I would expect on balance that more people would feel at home with a D-Pad style arrangement. OTOH the subsection of people who are n900 owners but not console gamers would probably be more at home with the n900 layout.

No doubt this is one of those ergonomics factors where you're not going to please everyone

EDIT: Third alternative: TV remote, which i expect people would use with their thumb in their dominant hand...