In almost every case, in fact I cannot think of any exceptions, bigger is better, and at a difference of only 50.00 you might just as well get the 180. The fact of the matter is, while both a 150 and 180 is the same length, you get more width in a 180, and width is an important aspect to consider with any fish you keep. It's a bit foolish not to consider extra width I think, not geared at your input but rather another answer.
As far as freshwater or salt, Andrew brings out a great point. Salt water fish have the most beautiful colors to them I think and I do have a 180 African freshwater set up among my other tanks, but they cannot compare to the colors Salt water fish have. However, getting live rock or coral, the proper lighting to sustain it, sand, and having to get the proper salinity, it is more work and quite a bit more money to run a Salt water set up. Ideally then I'd say, with respect to your wallet, go freshwater, especially as well if you are bringing up prices.
What to put in the 180? Wow I mean you really have a whole world of options. You could build up a whole world of schooling fish in a tank like that. Most standard 180s like mine are 72 x 24 x 24 and you could put in about 5-6 groups of different schooling fish in proper schooling numbers, and build up a community of bottom feeders as well.
You as well are open to almost every kind of cichlid out there as well. Some actually would get too big for your tank, such as the Emperor cichlid of Tangyangika, but most could work. I have a mix of Haps, Mbuna, and Peacocks in my 180 with a pretty good range of colors and aggression, and as well I have a Malaysian Golden Jardini in with them. Not the normal mix that's for sure.
You could go with a heavily planted tank, or you could go with sand, so many different choices. Being you have a goldfish tank, I'm not sure what would interest you. If you want something flashy, go schooling fish with large numbers of species, if you are looking for something more exotic and usual, look into African cichlids, if you are looking for larger sized fish, maybe South or Central American cichlids are your way to go. If you are looking into expensive fish, or rare hard to find types, Victorian or Madagascar African cichlids are your ticket.