For simulating a busy tower environment, the only thing that comes close is a 360 degree sim. However, to accurately represent realistic traffic, you'll need to spend £5m a piece, if not more, and have a staff of 10-20 running it (double that if you're going to use this one sim in shifts......)
Oh, and that's for four ATCO positions.
Synthetic ATCO training cannot be compared to synthetic pilot training. Teaching someone how to operate an aircraft requires no extra personnel other than an instructor sat next to the trainee.
Until direct voice input matures and the technology becomes affordable, they're just not comparable.