Some say that the 390 is better than the AL391. I guess it's come down to personal preferance. The main differences betweeen the two are. The 391 has a little more complicated gas system even though they are similar. The 391 has a bolt lock lever on the left side of the receiver. The forearm on the 391 is much slimmer. The receiver is rounded in the back on the 391 as opposed to the angled receiver of the 390. Both have an automatic magazine cutoff and weigh about the same.