Now I'm just trying to figure out which will be fully compatible in OS X and net a good perf/$ ratio.
Here is my advice if the choice is difficult for you.
1) Go look at the games you play and see how well CF/SLI works in them. For example, if you play a lot of GW titles, you are very likely to get better SLI support on Day 1. If FC4 and AC Unity are on your list, then go with 970s for sure.
2) If custom display resolutions matter, NV is also a great option.
3) Do you want the ability to have the cards turn off its fans off at idle or in light gaming? An R9 295X2 cannot do that.
4) Can you live without PhysX? Some gamers who love Batman Arkham series or Borderlands don't enjoy those games without PhysX.
5) Do you see yourself buying a 4K HDMI 2.0 monitor in the living room and using a PC with it in the next 2 years?
6) Side-step and warranty. EVGA has good warranty and step-up program. You get a minimum of 3 years vs. I believe just 2 on the XFX 295X2. EVGA FTW with 1317mhz Boost is
$335 on Newegg.
7) Resale value and ability to resell as easily. I think it's easier to sell 2 decoupled GPUs than a single one with 2 GPUs onboard. To be able to resell the 295X2, you'll really want to find someone who wants to deal with CF and a power hungry card. I think it'll be easier to offload each 970 for $175 in 2 years than to sell an R9 295X2 for $350.
The more difficult choice are
after-market R9 290s for $480-500 vs. GTX970 SLI for $660. Now, that one is a
much harder comparison. Stock vs. stock performance is 94-95% as good and you have $150+ for Steam games, an SSD bump. If you are looking for max overclocked performance, in your small case 970 SLI > R9 290s. Something like MSI 970 Gaming would still overclock well and run quieter than after-market max overclocked 290s. Not sure if this is worth $150+ extra to you though.
Anyway, to help your decision, imo DX12 is less important than the other factors I outlined.