I would say the 2 mp10 due to the fact you can have different flow patterns instead of just 1 mp40. Even though an mp40 does move water 3x more than a mp10. I guess it all depends on what you're going to keep.
I was running two MP10's on my 40g breeder and it was about perfect. I would definitely suggest two MP10's over one MP40, especially if your tank is narrower than 18" front to back. You will have more flexibility in working with the flow.