I don't think they would have been as popular, and yes they would have likely broken up.
Their music, however, might have been better. I think that it would have consisted of the thrashy Hetfield rhythm playing, but with the excellent Megadeth-like melodic riffs (something that makes them a bit more interesting to me). It's certainly an interesting subject to think of.
Just think of Cliff and Dave both playing solos on a Metallica album!