Ssj2 is much more powerful. Ussj is simply a pathetic form. Cell could do nothing to Gohan as ussj (his free punch on Gohan was tanked), yet as ssj2 there was a climatic battle.
USSJ has a greater multiplier in my books, but it becomes less effective (the multiplier decreases) as one increases their SSJ power. I think that MSSJ Goku would have got minimal to no power boost if he had buffed his muscles against Cell.