Right now I have a 55 gallon FW tank and eventually ( years from now) want to have a saltwater tank. How much more would I get out of a 75 gallon vs my 55 gallon for saltwater? If I did saltwater I would definitely want a reef and lots of fish.
75 is better but I think saying you would be disappointed with a 55 is a bit biased. I have a 55 and I love it. Heck some folks are rocking 20 and 30 gallon salt tanks. One thing you will notice is that most people are going to tell you bigger is better....deeper is better. But better compared to what? Also will it be better on your wallet? Take into consideration everything. If I had a deeper tank I would need more rock and that stuff ain't cheap. More sand. More water to take out in water changes. More money for salt etc. bigger skimmers. The list goes on and on. So I say hey if you already have a 55g. Then use that. Spend your money on other things. Like a very nice skimmer, reactor, lights, and power heads. Look on this forum, you will see some extremely nice 55g tanks.