Both are pretty good lights, very durable. I had an aluminum for a number of years. There were lots of dings in it from being dropped, but I think the poly would have been just as bad. You can also get the poly version in yellow, and I think orange. They are better to hold when cold, since plastic doesn't get as cold as metal. The rubber grip on the poly version isn't as good as it used to be, but still a better handhold than the metal body of the standard aluminum.
The XT is an aluminum body, with the switch placed at the tail, more like a SureFire, and I think a rubber grip.
Whichever one you get, spend the extra few dollars and get the fast charger. I hated having to have my light out of commission for the 10 hours it took to charge it.