Is there a tangible difference between these two?

RRA has high rear shelf near the rear take down pin - no RDIAS for you (99.9% don't care).  RRA has extremely tight rear section near the rear take down pin - many have had to "modify" their uppers to fit them into the lowers without a mallet.

Bushmaster no high rear shelf - see above comment.  Bushy receivers accept nearly all uppers without "mods" - "if you like a sloppy upper to lower fit".

RRA is a better value, dollar for dollar.

Finish comparison between the two is a wash.

Good reply - I appreciate the 'no flame' response.  Thanks.

I am looking at building my own baby from the ground up including:

2-stage trigger.
Skel. buttstock
RRA upper and Lower
?? 24" vmt bbl, heavy contour.
100% optics-based platform
Free-float alum. handguard
.223 caliber

Beyond that I'm flexible.  LOL.  Probably will have the 'tactical' grip.  Tried one in a gunstore and loved it.  Looking into an 'upgraded' bolt carrier ass'y.  Not 100% sold on the skeleton stock - it's as much a 'look' thing as anything right now - looks like it'd go well asthetically with the grip - but I don't know how well it'd work in a 'reach out' type rifle.

I know AR's are interchangable on the top end.  Wouldn't mind flexibility but not stricly required.