I don't think there are big advantages, other than your personal preferences. I recently tested both systems on my Nexus 4 and they both work very well at this point (no GPS yet on N4 with SFOS, but N5 should be fine according to https://wiki.merproject.org/wiki/Adaptations/libhybris).

In my opinion there are more apps and customization options (3rd party) available for Sailfish OS (no Android support of course), the system offers more choices (accounts, settings) and generally feels more mature. There can be some UI scaling "unbeauties" on SFOS due to the high display resolution, but upcoming v2.0 should fix all that.

OTA system upgrades are not supported yet on Sailfish OS and might break stuff (like certain camera features), so a reinstall is recommended and can be annoying. Otherwise Sailfish OS is best supported on the Nexus 5 and was also used to show upgrade 2.0 at the MWC.

Sailfish OS runs very fast on my Nexus 4, while Ubuntu phone had some lags here and there (probably not the case on Nexus 5).