You'll find that a single DXD12012 will sound a little deeper than two DXD808s and have a little more scale, but it is surprising how punchy and tight a pair of 808s are. The good thing about the 808s are their tiny footprint, and regardless of whether you use one, two, three or four stacked (huge benefits), the footprint stays the same. The good thing about the 12012 is that a second brings far greater benefits than a second 808 (obviously), and because you start with one, you can take the easy upgrade path further (and you don't waste money selling old subs on), if you know what I mean. If you want depth, the 12012s are the way to go. If you want to sacrifice that a little and gain punch, the 808s will be the way to go - although adding 808s will increase low frequency output. You can have a listen and choose for yourself, but either way will be seriously impressive. The subs are just as fast as the speakers, and serve as a true extention of them, which isn't something all sub manufacturers can claim/achieve.
I have the Q125s across the front and a pair of TriFX at the sides, with two DXD12012s. I'm using a Rotel RMB1575 power amp, which into the Quattros gives me about 500wpc - not that I ever use that much power! Nice to have plenty of headroom though