I'm a fan of both, but it boils down to this in my opinion...

Zakk Wylde has much more feeling put into his music.

Dave Mustaine is much more technical.

It's all personal preference on which is more important, feeling or technicality. They are both 2 of the greatest players for different reasons.

Kind of like comparing Yngwie Malmsteen to David Gilmour. Well, that is a drastic comparison, but it's feeling vs. technicality.

I have to agree about Zakks solo on In This River. That solo just doesn't fit the song at all...