As a dark lord my choice is clear, and Palpatine probably could have used the other hand. Of course, the third Dark Lord from the lineage of Bane would be more rival than ally and so that probably would not have ended well.
But in all seriousness I'm a bit of a 'git off my lawn' gal and so if the Empire were to piss me off I'd likely be straight off to a rebellion cruiser. However, frankly I wouldn't bother to take a position unless forced to. I could very well fight for the empire if that is where circumstances lead even if I was aware of atrocities. I tend to be that pesky 'gradual reform' voice, preferring to alter an institution rather than destroy it, and given the long odds of the rebels in the series I might deduce that there's little chance that the empire would go down anytime soon from the outside. Basically it all comes down to circumstance. All things equal as an armchair observer and assuming I was teleported into a galaxy far far away I'd probably seek out rebels first but I really can't account for what I might think if I were to live it in the moment.