We did both tours booked thru our ship, Star Princess, 2 yrs ago. The helicopter took you up to the glacier and you got out to walk around. The glacier was like walking on very, very dirty, icy snow on the ski slopes. It was hard ice and full of large crevices. You didn't stay long on the glacier before you had to leave.
We enjoyed the float plane over Misty Fjords much, much more. The view was spectacular and we saw mountain goats on top and landed on an alpine lake. You could get out of the plane and look at the view. It was very serene. Both are indeed different experences. Have fun!