Available floorspace is the biggest concern, I think.
Also, there's the question of lights, at least if you're talking about Normal Output fluorescents.
A 30" tank will most likely use 24" bulbs (in a 30" hood), so the tank will be slightly darker at the ends. However, 24" bulbs are pretty easy to find, so they're cheaper, and you'll have a lot of color choices.
A 36" tank generally uses 36" bulbs, so the lighting is more even on the ends. However, good (for an aquarium, and especially a planted aquarium) 36" NO bulbs are a pain to find (in my experience).