it depends what do you need.. Echo server is already fully developed and a lot of players have "crossed" the game, I mean they have all achievements and they are big levels, so it will be very difficult to break into the scene and it will take a long time.. and I can say population on echo is medium-low, because some players switched to Ilyzaelle server (mono-account server where you can play only on one account).. also, every few months Temporis servers are opened for duration of 2-3 months and then they shut down, so many people going to those servers too.. if you are nostalgic and want to play old dofus 1.29, I would highly recommend you to start playing on Dofus Retro servers that are open few days ago, there is a very high population at this moment and it's very challenging now to play because it is beginning and everyone trying to break into the stage, be the highest level, have the best guild, trying to drop gelano (there are hundred of players on jelly peninsula lol), some interesting things haven't even started yet, such as breeding, pvp agros, prisms.. I have personally left the echo and started a new dofus career on the Dofus Retro Droopik server and I am very pleased.. but as I already mentioned, it depends on what exactly you are interested in and what do you want in dofus, you certainly have many options.. whatever decision you make, you will not be wrong.
Enjoy!