First question:
Stag vs. Mega lowers

CMT Vs. RRA uppers

Where can you get cmt uppers? and someone said they saw a bushy chrome lined barrel for $150.. got a link?

Have no experience w/Stag or Mega so can't help there.

If by CMT you mean Continental Machine Tool, up in Connecticut, the only difference is whose side markings do you like better? Continental makes RRs lowers.

Denny, on the EE section of the site, sells Continenal uppers. Got one from him awhile back.  Decent quality, but nothing to go bannanas over. Had to deepen the bevel that the dust cover latch snaps on, so I could close the cover with one thumb, instead of needing two hands.  Also had to pull the forward assist out, and bevel the edge of the hole, to stop the sharp edges from wearing off the finish on the FA.  Good fit and color match w/the Bushie lower though.