First of all, Yadier will turn 30 this July, and as I've said before I tend not to trust the health of a catcher at that age. Not that there haven't been catchers that age that could play for a full season, but it's hard for me to feel good about their health. For example, before this past season, Buster Posey was being looked at as the catcher of the future for the Giants (Baseball America posted the Giants Top 10 Prospect list on December 13th, and had Posey listed as the catcher for them in 2014). However, a week before BA posted that, ESPN reported that the Giants planned to give Posey some time at 1B. Anyway, this age issue would be exacerbated by the fact that the deal would probably go for 3+ years.
By the way, I'm well aware of the fact that Russell Martin is not much younger than Molina (about 7 months). But, as I pointed out not long ago, I'm not keen on signing Martin for 3 or so years either. So that last part was basically a vote against both men, more than a "one or the other" thing. But, better the devil you know, right?
Secondly, I'm not sold that Molina would be a better offensive weapon than Martin. Yadier had a great 2011 season, in which he posted a line of .305/.349/.465 with 14 HR, but every one of those numbers is well above Molina's career averages (.274/.331/.377 with 9 HR). On the other hand, Russell Martin put up a line of .237/.324/.408 with 18 HR. The BA and OBP were well below Russell's career averages, while the SLG and HR are right in line with them (.267/.359/.398 with 15 HR). You can take that to mean that Molina would be due for a drop in numbers thanks to moving to the AL from the NL (like Martin did), or that Molina will come down to Earth while Martin will come up (?) to Earth.
Lastly... defense. Man, I'm not a fan of comparing players' defense. I'm a numbers guy, and there is no statistic/metric that's good for such a thing. So I'll simply leave it at this... both men are excellent behind the dish. Hell, Molina might be better, but Russell has done a damn good job back there. So all this leads back to the "devil you know" saying.
There. Now when I hear a Yankee fan talk about going after Yadier, I can either link them to or quote this post. Feel free to do the same.