1.23 servers are much more common because more people have 1.23 than 1.24 (TSF), but also because more people can join. Using a program called Project Controller 4 you can tell 1.23 to join TSF servers and vice versa. The only problem is that is you use it to join a TSF server with version 1.23 and the server loads a TSF-only level, you'll get an error. That's just another reason why you see more 1.23 servers.