I always pic the 3rd floor...although there are some disadvantages:
Advantages:
1) a lot of apartment complexes have taller ceilings on the top floor...makes it feel lot more airy.
2) seem to be farther away from pesks (ants, roaches, mice, et.) in theory...but this is most likely just psychological
3) unlikely to have water damage.
4) most likely safer from burglary (again...probably just psychological).
Disadvantages:
1) BIG ONE: Pain in the ass to move in
2) heating/cooling may be more expensive...but depends on how efficient the apts system is.
3) having to walk up those damn steps each day (although could be construed as exercise).