the AM stands for 'All Metal'
So basically it has more metal parts. It also has a slightly different frame allowing for the installation of a different board - giving you a screen.
tbh, I personally think the LT is better as its slightly lighter and I don't think the metal is worth the extra. By the time you've upgraded it you may as well have bought a more expensive marker. I shoot an Etek 3 LT at the moment and have no complaints, its a great bit of kit!