My fiance & I have a MU XL & a Speed P8. I really like both, but I prefer the frame on the MU. I wish I had bought a MU P8 instead of the Speed.
Carrying the MU XL or MU P8 up & down stairs is not a problem. The P8 is perhaps a bit lighter when both are set up with similar equipment (racks, fenders, lights, etc). Depends mostly on how much weight you can handle. I carry mine up stairs all the time with a rack trunk loaded down with tools, lunch, clothes, etc.
I can't speak for the Vitesse, but either of your other choices will do fine for the things you list. Both are excellent city bikes. They handle very well including when loaded.
Between the XL & the P8, it is really more your preference as to type of hubs/gearing. Both are a similar price by the time you set them up with similar accessories. Thor at http://www.thorusa.com/dahon/2007.htm
has left over 2007 models.