If I were you, I'd get the 75
but I'm a saltwater addict. I also know however, that it's not a hard thing to become. Overall I feel that larger tanks take less work...once they're established they basically run themselves. (well, with live rock, live sand and proper filtration they do)
But, I also have a 200gal, 100gal, 90gal, 30gal, and 25gal saltwater tanks set up right now. (I have a 10gal, but I'm waiting on getting a mantis for that one)
The best thing I can say to do is look around at pictures and read a couple of books. I think that one of the most generally informative saltwater fish books out there is Marine Fishes by Scott Michael. It tells you basic tank requirements for the fish, shows you a picture and tells you general information on the 500 or so fish in it.
Before you decide which tank to get, look at all the fish you can so that you get the right tank for them. A lot of people will be perfectly in love with/content with a 25-40gal tank until they run across a tang of some kind (who require at least a 75gal tank) and then they wish they'd gotten a bigger tank...or they go and get a bigger tank