I think they're best used in conjunction with one another. Chess Mentor does a great job of helping me to understand certain positions and the key ideas behind them. Tactics Trainer is better because I usually don't have a GM whispering key ideas in my ear while I play :)
Chess Mentor vs TT.

Oh, I thought this was going to be about if Chess Mentor and TT played a game of chess, who would win?

Chess Mentor: "Alternate correct move, Bxc7 is playable, but it's not the line we're looking for."
TT thinks to self: "Oh no...one those chatty pedantic types".

Oh, I thought this was going to be about if Chess Mentor and TT played a game of chess, who would win?

I think they're best used in conjunction with one another. Chess Mentor does a great job of helping me to understand certain positions and the key ideas behind them. Tactics Trainer is better because I usually don't have a GM whispering key ideas in my ear while I play :)

What if identical twins were trained seperately...one using only Mentor, the other TT exclusively; which one would be better?
That might settle it.
Which has helped you improve your chess skills more? I vote for chess mentor. It explains the moves to you.