First, there's a good chance that neither of them work out that well. Second, there is a chance that just one of them will. Third, there is a chance that both work out well.
To me the chances that it's Mez or Grandal are very similar. Just being 1 year ahead of the other doesn't really mean much to me for the Red's purposes.
Besides whether Mez is the difference between Felix or Shields, say, I think the variable that says the Reds actually like Grandal better is actually just as likely as them liking Mez better. Or they just can't make up their minds. Who wouldn't believe that?
Nevertheless, trading one of them this offseason seems like the right thing to do when you consider:
1) Mez doesn't work out
and
Yas doesn't work out
2) Mez works out
or
Yas works out
3) Mez works out
and
Yas work out
1 out of 6 scenarios would produce regret, if the Reds trade the one who works out. Both values are very high, Mez's is higher. If the Reds actually like Grandal as much or more, they should definitely trade Mez for a great return if it's available.
If there isn't a good deal for the Reds, a "Carson Palmer" type deal let's say, I won't mind keeping both, either. I've said before that Grandal can be a stop gap, if not a real solution, at 1b in 2014, batting right behind Mesoraco, our cleanup hitter.