defytf2 center basically replaced lobbies, tf2center still serves for players that are really new to 6s so its not splitting us its more of a stepping stone, new players in mix come from tf2center/lobbies etc...they start in that and eventually make the jump to play with people who are more experienced.
Honestly this probably doesn't happen that often. TF2center 6s is a shitshow. People don't go into mumble, and off-classing lobby stomping is rampant. If that's your first time playing 6s you won't go "wow, that could have been fun if I was better" you'll say "these people are assholes and their game is stupid." When I started I had to start in TF2lobby and the only reason I stayed is because I had already been watching a lot of 6s and I was curious where I could play a game that didn't devolve into demos off-classing to huntsman sniper.
The only benefit to TF2center 6s is ease of use, or in rare cases people using it to set up their own inhouse games. Beyond that it probably does more harm than good for growth of the 6s community.
The best place to start is probably inhouse pugs. People there tend to all be on the same page and fixing/kicking a problem player is pretty easy. It retains the picking system so teams hopefully are balanced and it keeps the envornment friendly which you need to retain a new player. From there you should probably have a decent time, even if you suck. From there you should jup to tf2mix or scrims/low-level team play.