I have a .300 Whisper, and I'd vote for the blackout if they actually get commercially available brass.
There's no truth to saying that the Whisper is hard to adapt to the AR - it's as simple as any caliber is going to get, it's a different barrel / gas setup and you're done. (Same bolt, mags, etc.) BUT, there aren't commercial brass available (unless you count ordering brass from SSK), and it's proprietary, so if anything becomes available it will have an extra cost associated with the licensing.
If you're getting into the game right now, I wouldn't take the plunge unless you are prepared (at least as a backup plan) to form your own brass.