A billet 7075 receiver is practically speaking about as strong as a forged 7075. They use extra material to make up the strength difference. I like the looks and lines of a matched billet receiver set over forged - just personal preference, so that's what I have used.
Either way, billet or forged, I would stick with matched receiver sets. If you want to do a build on the cheap, then mix and match receivers, but don't expect anyone else to be impressed with your build when/if you ever go to sell it. Using mismatched receivers you are more likely to have a gap between your receivers and certain lines, indents don't match up perfectly. If you don't care about resale value, then it really doesn't matter. Heck, there could be a gap between the two receivers and your AR would probably run just fine. None of this really affects function.