I would go with the MP40. I think that you would really have to crank up the MP10's for a 90 gal tank. They would be working harder, hotter, noisier,and probably ware quicker. The down side with one MP 40 is that you cant get the cool back and forth flow. With 2 Vortech's, you can get some really cool flow patterns. Tough choice.
I have an MP40 and an MP10 in my 75 and they work great together, and I didn't have to drop a ton on two MP40's.
75g Mixed Reef, T5 Lights, 2 Vortech's, Apex Controller.