I've had two Demolition mans, but the theme really did little for me besides it being a Williams pin. On top of that, it is a nightmare and a half to shop properly and a very challenging game to keep working correctly. On the plus side, it is a fun shooting game with good sounds and the play is definitely more challenging than T3.
T3 on the other hand is very straight forward. It's also very reliable and easy to fix. Has a lot of unique features, such as the all metal ramps, backbox rpg cannon, great callouts from Arnold, great thumping sounds, video mode, light show, etc. It is a heavy ramp game, but they are fun ramps, and the modes are fun too. The assault mode is easy to achieve, and the final battle is not too difficult to get either.
One of the games greatest features is the bonus x is deep, and it holds/collects your collected items, (such as weapons) throughout the game for your bonus. T3 is also a great game to play after you've played a really hard pin, because sometimes you just want to play and feel good playing and T3 delivers that.
My vote is T3.