I think the person better equiped to answer this question is you, by trying both and doing the same tasks, and then comparing.

Both WM, don't use compositing so they present the applications directly on screen, what can be faster if you don't have a high end graphic card.

From what i read, it seems that Gnome Fallback could be a little faster, but there's not real data tests to prove it.

So my advise is, install both, run the same application (play a video for example) on both, and measure the times.