Honestly their is no problem in reliability from either M16A1 or M16A2.
The problem with the original M16 (not a1) was a poor choice in ammo (stick vs ball powder kinda stuff), the barrels were not chrome lined, the soldiers would just hold down the trigger like it was a lazer gun and so the guns would jam after 400-500 rounds of contentious fire...like any gun would! On top of this a nasty rumor spread that the original M16 was a mircle gun and that it did not require any cleaning - ever.
So you have soldiers in one of the most inhospitable environments (hot/humid/damp jungles of SE Asia), shooting a firearm in full automatic fire for upwards of 1,000 rounds, using a bad type of powder, you're gonna have issues.
The biggest problem that would happen would be the M16 after its first shot would jam, leaving the empty casing in the chamber. Soldiers in firefights were known to pick up AK47s in early Vietnam War because of this. In essence their firearm was turned into a muzzle loading plastic piece of crap and many times you would find soldiers trying to find sticks to poke out the empty cases that had become stuck.
When the government had the M16A1 created all the problems of the M16 were removed. They changed the ammo, they chrome lined the chambers and added little touches like the reinforced lower backwell (near the stock) and the forward assist.
The reliability from M16A1 to M16A2 did not change very much, if at all. Either one you will be happy with.
I love my M16A1 to be honest though. It has never had an issue with reliability and is so much lighter than the M16A2 or M4s I've tested or used. I think last time I weighed it, it was around 6.0-6.5lbs, where as my M4 with optics was 9-10lbs.