I have a tower 75 I'm breaking in as we speak. I don't have an MDS 78 and have never heard of a os 61sf. The tower 75 has more displacement, horsepower, and torque than the OS61FX, but the OS61FX does weigh more and costs only 45% more. I must warn you about breaking in the tower 75 on a bench though. It tries to make a wing out of my black and decker workmate surface, so you might want to consider concreting something down in the backyard. I doubt you would have that problem with any 61. Cheers!