I would absolutely say that Valkrowser Ultra Rock Beast is better. The water stealth pretty much makes it unblockable, as most good decks that have blockers run water, essentially making him a bigger Crystal Lancer. And the fact that it has 9000 rather than Doboulgysers 8000 is a pretty big deal. Valkrowser gets over Lancer, Bluum, Apocalypse Vise etc. it can also trade with Starnoid and Tanzanyte.
Doboulgysers is not a bad card at all in itself, but Valkrowser is better. If your gonna run a deck with rock beasts, i would definitely include water to make it more consistent, and for some early aggro cards like Emeral and Aqua Hulcus. I have never tried a dedicated Rock Beast deck before, but i would go with something like this:
x4 Blazosaur Q
x4 Phantom Dragons Flame
x2 Pyrofighter Magnus
x4 Valkrowser Ultra Rock Beast / Doboulgyser Giant Rock Beast
x4 Aqua Guard
x3 Aqua Hulcus
x3 Energy Stream
x4 Aqua Surfer
Im sure this is not the optimal build, but it was the first one that came into my head.