The 55 gr is a little cheaper and will work for just about anything that you need more than likely. The 62 gr holds its energy farther out than the 55 so it hits harder than the 55 gr say at 200+ yds. If you plan on shooting over 200+ yds I would recommend the 62 gr because the wind won't toss it around as much as the 55 gr. Another recommendation is to sight your rifle in with one weight and stick with it because there is a slight point of impact change. Overall if you are just pinking short range just go with the 55 and save some money. But if you're going to hunt or shoot longer ranges with you're rifle just go with the 62 gr.

I'm guessing you mean M193 versus M855? M855 is the 62 grain "green tip" round. Both have their pros and cons, but for "general use", which would mostly consist of practice, M193 would be my go-to option. As for most scenarios which you'd be in with an AR15, M193 outperforms M855. They share very similar velocities and ranges, but M193 will fragment much more readily in soft unarmored targets. I like IMI brand, as that's first run ammo that their military uses.

With 1/7 and 1/8 twist barrels both should perform equally if you have a 1/9 twist go with the 55gr otherwise get the cheaper of the 2

I'd be interested in this answer/opinion as well. I'm on the fence about getting an AR. I got a Savage Varminter Low Profile in 223 w/ a 1:9 twist. So if I may be able to simplify my ammo needs. I don't have any exp with those 2 types of ammo, but so far, out of the 6 brands I've tried, my rifle likes the 62 gr Hornady Black ammo. I haven't worked up to match grade yet.

it didn't say that it had to be in 5.56 mm, I would cast my vote for the 6.5 Grendel and the Hornandy 123 Gr SST bullet.

55gr limited ar experience have had no issues