For solo I would vote for beasty. I see more kills per hour with beasty mowing through light blues. The shaman is more tied to mana pool, using root rot at high levels. My beasty uses slow, incapacitate on heavier hitters like in sunderock springs, and the disease dot line. At higher levels I avoid the ice nuke and poison dot as they are resisted a lot.
Healer mercs have changed the solo+merc game so much that you could tank with shammy using maw line.
So play both and see which one you like better.
No PvP experience.