If I were you, I'd definitely sell my HD7 because of the hardware improvements to the Mazaa and you'll always be able to test some app on your mother's phone before releasing it just to be sure - but I wouldn't think there will be much errors if both get the same update after all.
But very much so, I'm sure all people around here are very eager to know if there are any (interface) changes or additions in the RTM build! I don't know about any Microsoft issues if you'd tell. I think you yourself know your agreements best by entering that competition
. I think XDA would gladly feature some article about it if you'd be so nice to write one!