I have extensive experience with the G3A3 and G3A4 (and the updated norwegian version with handguard rail, optic rail, two position collapsible stock and Comp M4), as well as having shot the HK417 quite a bit (we have the 20" version).
If the choice was between those two models as a battle rifle I would choose the G3 every day of the week and twice on sunday. Upgraded version of course. I just find the HK417 version we have to be clumsy and unwieldly compared to the G3. If I tried a shorter 16" version of the HK417 I might change my mind though....but for now, the G3 is the winner. And what is supposedly wrong with the roller-delayed blowback system? During my 7 years of daily use, including a 1 year deployment, I found it to be extremely reliable and a good shooter. Only drawback is that it was a hassle to clean the barrel extension area. Now don't get me wrong, I love my HK416, but wouldn't hesitate to bring a G3 in to combat.
One other aspect of the HK417/MR762 is the accuracy capability. I do not know what the factory accuracy is on the MR762, but our HK417s are 1 MOA guaranteed. We have seen weapons and shooters accomplishing sub MOA groups at various distances. If you were to run your MR762 as a battle rifle, with long sequences of fire with a significant amount of ammo, and subsequent heating of the barrel, that kind of accuracy will degrade rapidly.
We use our HK417's for our snipers (not DM's), and only guys who graduate our level 3 course get issued one as a personal rifle. However, one guy who was not a sniper got hold of one, and started using it as a battle rifle and running reaction to contact drills, australian peels etc. Suffice it to say, the head sniper instructor for the Norwegian Defence Forces was less than impressed.
As far as issues go, we had a minor ammo problem, with rims being torn off during extraction, with the remains of the casings being stuck in the chamber. One particularly hairy incident in Afghanistan, but no one got hurt. Not going to go into specifics about the ammo type, suffice it to say that the issue is fixed with the ammo we are using now. We have had no issues with the AGR's on our weapons. The biggest issue has been with the first batch of sights, S&B PMII 3-12, with the filament freezing or something to that effect, and not moving when making windage and elevation adjustments in cold weather. We have new scopes now, same type, but upgraded.