I think if you are practicing real world scenarios and training then recoil weight and same size of the gun is very improtant so I think first should be one in 5.56 becasue regardless it will be cheaper especailly if you reload. Where I think a upper like a 22 lr can be very beneficial is with basic skills and in getting the basic movements down.
I have to agree with Tim; 5.56 is the way to go for practice if it's too expensive to shoot strictly 6.8. I tried to use a dedicated .22LR upper for basic drills, but the lack of recoil totally negated any trigger control training for me.
I was wanting to see what some others did for practice. I've always used imported 5.56 (sounds better than saying Wolf) or inexpensive M193 when you could find it. A store here used to have Federal and Winchester 55g M193 at reasonable prices.
I've seen Ciener 22 conversions, and have seen Spike's Tactical selling their 22 Uppers and wondered what they were like. Trigger time and sight picture drills would be the same. Afterall most instructors say dry firing is good for practice so why not a 22LR?
Then I spotted a case of 5.45X39 ammo for $139.00 and thought that one of those uppers from S/W or someone else may be interesting for practice.
I'm not reloading yet, so I'm online looking for ammo deals like most of us.
OF the two 22s conversions get the Spikes. One he is a site sponsor two he pust out solid ear and is there to help fixes and bumps in the road that might occur. It is also a great add on if you have a kid that just really want to have fun so you don;t break the bank and he still gets to shoot he ubber cool AR.
I shoot 556 for practice since the base furnishes it along with my 308.
I have the M261 (military) conversion kit in one of my AR's that my kids use. They get tired of it pretty quick and either want me to change the bolt out to 556 or shoot a different one. I like the conversion kit but it doesn't work the same in all the rifles. It fits tight in some so I keep it in a certain one all the time.