I am about to buy a stripped lower, either Rock River or Plikes Tactical. The local gunshop I like to support has these in stock requiting no shipping or FFL transfer fees. For all the other parts, I want to buy from PSA (Palmetto State Armory). Question - Never having built an AR, will I have issues fitting PSA uppers, trigger groups, bolt carrier groups, etc to Spikes, or Rockriver lowers? What should be most concerned about? What should I stay away from in terms of parts for the lowers I am considering? Of the two brands I am looking considering, which is a better, more reliable lower?