It's really tough, but I'd go ahead and pick Sanchez mainly because he's more versatile than Jovetic.
I would disagree with that. In our current system Jovetic is more versatile IMO, because he could play SS or CF; Sanchez would be limited to the SS. If we played 4-3-3, there's no problem fitting either of them into the lineup.
Sanchez has speed which we could definitely use up front. Jovetic has more of a goal scoring presence in and around the box, though - and that's something I think we really need. I'm not convinced that Llorente is going to be clinical enough up front, but we'll see what happens. I like Jovetic's size compared to Sanchez as well, and I think he'd be more useful for buildup play. Also think that he has the higher ceiling.
Alexis would be a very solid addition too, but Jovetic for me.