At least, it's been the question people have been asking me quite a bit. Given we haven't announced the availability of the XNA Framework yet, but we have announced that MDX2 is a "technology preview", but will never ship in a "released" mode, the answer seems relatively straight forward to me.
If you are shipping something that needs to be "released" by a particular date, you'll probably want to be using MDX1, as it is already released and available. If you have a project that isn't tied to any strong dates and you can wait for us to ship the XNA Framework, you can stick with whatever you're using right now, be it MDX1 or MDX2.
The question is somewhat more complicated if you fall into the former, but the "particular date" is say a year out. Then it becomes more of a business decision for you. "Can I risk sticking with MDX2/XNA Framework with no guarantees it would be out within a year." If you can't, then you should probably be using MDX1..