I am pretty sure that the Rotor adapter you mention will not allow you to use a TRUE BB30 crankset on a non BB30 frame.
A True BB30 crank does not have a spindle long enough for an external bottom bracket.
I understand that it's really only the Quarq S975 and Red cranks and the Rotor 3D+ that I could use on the BSA frame with the adapter. As I'm primarily concerned with using a powermeter between my current BSA bike and a future bike that most likely will be BB30/PF30, etc., I'm okay with that. Ideally, i'd like to have a BB30 Quarq and a GXP quarq, but seeing how hard it is to convince the wife about n+1 bikes, I think I'll have to go with one or the other.
There will be no difference in Q-factor. BB30 vs. BSA doesn't really have any relation to Q-factor contrary to what many people say on the internet. Mostly Q-factor is related to chainline and how close the manufacturer wants the crank to the chain when you are in the 53/11.
With that being said, the GXP Quarq will fit on pretty much any bike but the BB30/PF30/BBRight frames will require an adapter. Likewise, the BB30 Quarq will also fit on most frames (except for Trek) when using a RotorBSA30/Zipp Vuma BB on BSA frames.
Edit: Long story short...I'd go for the BB30 Quarq if your frame is BB30 and aren't planning on riding a Trek.
I guess my question then is whether the GXP cranks and the BB30 cranks have a different q-factor. If the S975 BB30 is designed to fit on both a BB30 frame and a GXP frame, meaning that the spindle length is great enough to fit on the GXP frame with a RotorBSA30/Zipp Vuma BB, then when using the crank on a true BB30 frame with shims, the q-factor would still be the same. But using a true BB30 crank, which would have a shorter spindle, on a BB30 frame would result in a smaller q-factor and cannot be used on a BSA frame. Correct?
Also, I've seen a lot of people complain about BB30 bearings creaking, but I haven't heard anyone complain about GXP cranks w/ adapters in BB30 frames. Would it be smarter to just go with a GXP Quarq and use GXP->BB30 adapters in a BB30 frame? With the BSA30/Zipp Vuma, I'm not using adapters, so I'd figure that would be better in either scenario. Thoughts?