I think you would be crazy not to use a Stag lower. First off you already have a stag upper and LPK, maybe I'm just anal (and I know it doesn't matter in the grand scheme of things) but I like to match up as many parts as possible on my AR. That and at $99, Stag lowers are one of the best deals out there.
Stag lowers are made by CMT who also makes RRA lowers. Judging by my RRA lowers (compared to others I have or have seen in person), CMT does a very nice job (One of the best IMO) milling a lower. The only one that may be prettier is Mega but I've never seen one in person?
The only down side to the Stag for me is I can't stand is that damn ugly deer head logo! Me personally, I hate that logo so much I'm willing to pay more to buy RRA lowers (even though they are pretty much the same lower) just for the better rolemark but even I would suck it up and buy the deer if all my other parts were already Stag. So to sum it up...
1. Best match for the parts you already have.
2. Probably the best value (Price vs. quality) out of all of the AR lowers out there.
I wish I had bought more RRA lowers back when they were $99 each