The advantage from a wetsuit differs from person to person and from one wetsuit to the next, but 5 sec per 100 is pretty close for me. For 750m, I'd wear a wetsuit if it's allowed, but at that distance and temperature it's very much up to personal preference. As for the tri suit, as long as it's tight and specifically made to be worn in the water (for races....I've seen some full-body training suits that will cause a lot of drag), it should be fine. Most races in FL aren't wetsuit legal and that's what I normally wear. If you don't have one, you should expect to spend well over $100 for a nice one....Fastskins are like $250 I think and are really fast in the water. If you don't feel like dropping that much money, tri shorts will be perfectly fine.....just don't forget to pull the drawstring plenty tight.