Although I didn't see either production, I do know from research for my undergrad honors thesis that Mostel often stopped the show with improvised direct addresses to the audience. I know that direct address is written in to the character, but he would add more, and wouldn't always stay in character. Apparently he would add comments to the audience about current events and baseball scores (so I've heard).
Mostel also did this in
A Funny Thing Happened..., but given the nature of that show (the characters know they are putting on a show), such comments actually added to the comedy. But it seems to me that breaking character in this way in
Fiddler would ruin the experience of the show.
Based on this information, I would have to say that had I seen both performances, I would probably have preferred Molina's.
On the road of life, do not pause for suicidal chipmunks who freeze in your headlights, seeking death by your tires...