People have been saying more and more that the 5.56 is anemic or underpowered or a .22LR on steroids simply because they heard it somewhere. .22LR's are usually 40g @1100fps. 5.56's are 55g @3200fps (picked the most common loading of each). That's some steroid! Try hitting a man sized target at 300 yards with a .22LR, then try it again with the 5.56. In the first instance the cartridge really isn't up to the task, and the rifleman damn well better be. In the second instance, the cartridge is very capable, and the rifleman still damn well better be. Seeing a commonality? THE RIFLEMAN!
It's all about shot placement. Always has been, always will. That being said, I'd buy the one you can afford to practice more with. In this case...5.56.