when i was stocking my first tank, the guy in the shop (whom i also now fully trust having gotten to know him) said "if you have a big enough tank, why not go marine for your first tank?" to which i responded "ah, its only a 20 gallon, not good enough for marine on a beginner level."
I discussed very briefly a bit more, but he was saying a simple marine setup in a 4 foot once setup and read up on the equipment is very easy to keep, even more so that alot of freshwater fish (except for the simplest beginner fish.)
was quite amazed by it.