If you are looking for the feel and weight of your .223/5.56 AR, the top choice for me is the Nordic.
http://shop.nordiccomp.com/NC22_c6.htm
These upper use Black Dog magazines and go right onto your AR-15 lower. They are utterly reliable which is a huge benefit in the training realm. Accuracy is good, but they are not tack-drivers. I know that TacSol recently redid their uppers of the same type and the reports are that they have improved.
A large number of the top tier 3Gun competitors use these for training.