I debated this question myself back in early December. I had the choice between the Moto Droid and the TP2. I ended up with the TP2, as I have a slew of applications which I use and am so familiar with it. I got my Wife the Eris. So, having both of these phones, I am glad that I made the decision that I did. I know that the Eris is not the same as the Moto, but I do feel that Android is still a little behind in stability of platform, from the app. developer side, anyway. Please, no flaming on this aspect. It is JUST MY HUMBLE OPINION. I know that there are some issues with some apps only working on certain versions of Android. This said, I must say that Android DOES have a lot to offer, and I know that, for myself, I am happy with WM6.5 at the current time. (Still wish WM7 would get released already!) I must point out that I have been using WM for about 5 years, so I also have a little bias in that I am so familiar with it.
Don't know if this helps you at all, but it is just MHO.
Good Luck, and I am sure you will be happy which ever way you go!