If you really want midlength you should get it. Even if YHM is good quality you really can't compare them to BCM, they're completely different companies with different products. If you compare closely there are lots of differences besides gas system length.
Anyway, it sounds like you're in no rush so why not get a lower and build it up the way you want it and keep looking at uppers, no doubt you'll find a midlength configured the way you want it. I agree it's nice to have matching upper/lower receivers but it's not that big a deal. My ArmaLite has matching receivers but my other two non-matching ARs are actually better rifles.
Don't forget to look at PSA too.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek
Last edited by Quentin; 07-15-2012 at 10:24 PM.