Ummm, how did you kill a 400xpr? And have you tried contacting Dremel for service? They're pretty good about it generally. The 400xpr is a much more powerful tool than the multi-pro, and pretty well built. Maybe you got a lemon.
I'm guessing you mean Foredom for the other brand. If that's the case, they're good stuff. Basically commercial grade rather than consumer like the dremels. They're popular with a lot of craftsman and fabrication shops because they've got more power, last a very long time, and have a ton of options. Their flex shafts and handpieces are designed for a lot more power than the dremel versions, and like the motor are made for commercial use. Foredom's not cheap, but you're getting what you pay for and then some.