Personally, if it's a choice between the 2008 mu p8 and speed p8, I'd go with the Mu.
The Mu frame hindge is a very nice upgrade. I prefer the faster lighter tires (some might not). I like aluminum because it doesn't rust (steel requires more attention). I think the Mu looks a little better.
I got rid of my downtube IXNS for a mu p8, a much better bike.
I have a 2007 speed pro, 2007 curve SL, and 2008 Mu p8.