Montana also played when the defense could do bad things to QBs and receivers. OK, compared to now Young did too, but go back and watch some of the hits Montana and other QBs took.
Also, consider how bad SF when Montana and Walsh first got there.

In short, the NFL hadn't become the passing league it is now, and Young had a pretty good roster when he took over for Montana.

Young was great too - he's in the HOF for a reason - but I think it's Montana over Young.