The QRx 218 is a great sub, that's a great choice. We had 12 212's that we used for mains and 8 115's that we used for wedges, so I have experience with those boxes as well. The 115's sound good, get pretty loud, and have solid low end. The 212's get really loud, and have a little less low end. Neither box has the mid throw of the 153, is there any reason you're not looking at those? The 153 doesn't get as loud as the 212, but the mids kind of drop out of the 212 when you crank them that much anyway, so in my mind it's kind of a moot point. They're all good sounding cabinets, but the 212 needs more EQ in the high mids than the 115. The 115 needs some EQ too, but not nearly as much as the 212. The 153 needs very minor EQ, if at all.
Personally, I'd take the 212's rather than the 115's, and the 153's rather than the 212's, even though they're not quite as loud. The 153 is the better sounding box, but not quite the powerhouse the 212 is. That's just my opinion though, not the holy word. If you like the 115 best, I won't hold it against you.
We use QSC power on everything, which works pretty well. We had the older PLX 3402's on lows, one channel for each 212, and two channels of a 3402 for 6 horns. 4 amps for 6 cabinets. We had one channel of RMX 4050's for each sub, which were either MTL-1's or QRx 218's depending on the day. PLX 3002's on lows for the 115 wedges and 1602's on horns. We switched to 3402's on lows when we went to XW wedges, which helped a lot. I would have preferred EV power, but for the money, QSC works really well. RMX 4050's are awesome on those subs, it's worth the extra weight.