I used two mp10s in a standard 90 gallon tank for a while just because I had them and I like them better than the koralias that were in there before. They aren't ideally suited for a tank this large. I was able to run them at full throttle without any trouble, however, and was surprised how much water they moved around. I have since upgraded to two mp40s. I wish I had the clearance behind my tank to run the mp10s facing forward on the back wall in addition to the mp40s.
I personally would prefer to go 2 x mp10w on a 40B over a single mp40. With two powerheads, you'll get a more gentle push/pull instead of a REALLY strong undertow pulling everything in one direction and a REALLY strong stream pushing in the other. The cost is roughly the same, get what you want. If the 40B is a temporary tank, and you have plans to get rid of it soon in favor of something larger, definitely go with the mp40 though.