Well, they're completely different styles, but here's the breakdown:
Older Opeth: Had a very evil epic feeling to the music, Mike's vocals were fucking awesome, it's not that he's developed his growl into what it is, it's that he consciously made a change in heavy vocal styles, and I always thought older was more epic. But this also did have lots of songs that sounded the same in a way, with all of the dual harmonies, and that stuff, and some boring interludes sometimes too.
Newer Opeth (excluding D1 and D2) is very innovative, tries new things, and usually they sound great, Mike's CLEAN voice has gotten much better, and this is where the epicness of the music comes, the dynamics of the death metal grunt and his now great clean voice. The riffs are a lot more awkward and unpredictable, solos are more scarce, but have more emotion to them, drums are MUCH better. MAYH is my favorite Opeth album, because I believe it blends the best of both Opeth's it's very dark and evil, yet with great new riffs and ideas that they put to use today.
So I voted new, because I dont think they'll ever top MAYH or SL.