Lots of options out there, so your best bet is to check out and demo what other guys on the range are using. What works best for me, may not work best for you. The best way I found to get around the "which bag" question, was to order up some cordura, webbing, hardware, and fill, and begin making bags myself that mimic the dimensions of the plethora of existing bags on the market. Ended up making 8+ bags for the cost of a pump pillow or game changer...and finally settled on the 3 bags that work best for my needs.
Since that doesn't really answer your question though, im running my homemade TabGear Str8 laced for a rear bag, along with a diy pump pillow and gamechanger.
good luck!