while i totally agree with the whole FW/SW points brought up so far i must point out that if you have any doubts of your desire to maintain a fish tank.... keeping a planted FW tank will give you some insight to lighting, supplements, the nitrogen cycle, testing, water changes... the confidence to make the step into SW.
if you decide to start with FW (for whatever reasons) get a 29 high so you can use it as a sump later with your SW.
from personal experience i *love* my planted tanks. they have so much obvious life in them. in my book a reef tank is the ultimate cool, a planted FW is the ultimate calm.
-ashley