So far, EE is in a good overall state. Still a lot of players on both servers, lots of new content was added so it's pretty fun, yeah.
About the AP question: Well AP buyers of course do have an advantage and achieve things faster than non-buyers. Nevertheless the gap between spenders and non-spenders is smaller than in many other games. Overall, Eden Eternal offers a good bunch of opportunities to gain both gold and free AP items ingame to keep up.
I think both servers are equally populated atm, how many players there are...one can only speculate but I'd say "enough to find parties", which is the important thing. Sapphire is the "older" server, merged from 3 servers. It has more experienced and high-end players. Diamond on the other hand has more players in the higher mid levels (50-65) but is slowly getting to the state where Sapphire is. Both servers have their advantages so it's really up to you where you go eventually.^^