I have owned both the XT and the AIM Light TG, plus their datalogger. The XT is small and simple. Nothing to it and it does it's job.
The AIM has added value in that it will display a graph much like mylaps.com does via your AMB transponder. The AIM will also display two times simultaneously. Your current lap time and either your best lap time or the previous lap's time.
The display is big (I also recommend getting the backlit AIM lap timer) and the buttons are easy to push, even with gloves on, unlike with the XT. (There is just something better about having rubber buttons that stick out so you can actually feel that you are hitting them.)
The AIM lap timer also has 3 different modes - Practice, Qualifying and Race modes. Just like with a pit board you can set up your timer to count down laps, so if you are doing a 20 lap race, you'll know you are on lap 18. You can set up the timer to be used like a count down watch as well, so if you are doing a 20 minute practice or qualifying session, you will know if you only have 3 minutes left.
Basically, the AIM Light TG lap timer is the ultimate in lap timers and goes for around $250.
Whichever lap timer you go with, make sure to get a lap timer holder for $20 from www.laptimerholders.com
Here are pics in my garage last night of my setup and what the timer displays CAN look like. (Sorry, it's dark):