For me, I guess it is a matter of easing Ben Arfa into the side and possibly easing Best out. We are all aware of Ben Arfa's quality and that he should be in the side sooner rather than later. For now, I would continue to start Best and then, depending on the what's happening in the game, bring him (or even Ba) off for Ben Arfa (if we encounter, for example, another Wigan situation where attacking-wise, we're hardly doing anything).
But on the other hand, the Ba-Best partnership seems to be going very well for us. They seem to be slowly developing an understanding (as you should with strike partnership) and that can only benefit the team imo and so (for now anyway) I wouldn't want to see it being broken up. Best's hold up play has been important for us so far and he does give his all in every game so it would be harsh to see him dropped from the side. I would say see how he does in the next couple of months or so, if there is a drop in form for whatever reason, then Ben Arfa should be given a chance.