The high pop servers are generally any of the legacy servers- the ones that existed from the 1.0 era (well, kinda, they have different names now). I made alts on other servers before (either just to mess around with or to track down friends I found on the lodestone) and I've always been surprised and disappointed by the lack of people on non legacy servers, seems the only places I would see people would be around main aetherytes, and they're all afk.

There will be more crowding of course, but it's far easier to find people to do stuff with. I don't think judging market prices by server population is particularly easy. Some things are more expensive just because there's more gil on the server, some are cheaper because there's more people to provide them. You will at least have an easier time finding items you want on the boards.

Community wise, I haven't spent a lot of time on other legacy servers, but I find Hyperion to be really good, and I've heard people say (sometimes on various forums, sometimes people that have transferred there) that the server is pretty friendly in comparison to some other servers. With that said if you ever find yourself there, I have a very active and social LS and FC who's main focus is helping newer players learn and catch up, so if you're ever interested send me a tell