I look at it this way, there will always be something new coming out. Also, unless you have an iPhone you never will know when an update is going to be pushed to your phone. What I do know, is that my HTC Arrive is old (small screen, physical keyboard, heavy, no ffc, so so camera). I also know that besides Sprint's pricing (which they just changed how they billed employer discounts to screw more money out of you), the company is horrible.
So if I want a better Windows Phone, I need to go to a company that supports the platform and get the best device they have now. That leaves me with only this choice: AT&T and Nokia 900. I cant base my decision on unknown variables, I played that game with my Epic 4g and paid for it.