it's hard to call it better
both bikes are great, have not tried the Curve SL but the MU P8 is a sweet ride... if you are planning some long trips the MU is a better choice IMO. The Curve SL would be (maybe) easier to carry.... I have to mention i have test ridden the Curve D3 and it was very nice for short rides while for me living in Toronto 3 speed folder wasn't enough.... the 2007 Curve SL equipped with the SA 5 speed hub so it should be a pretty sweet ride.
My only concern with the Curve was how to fix a flat if i am in a middle of nowhere, with the Mu there is a QR and all i need is an extra tube and i can work on the other tube at home, while with the Curve (and i am not sure about the front wheel) the real wheel doesn't have QR... well you can always catch the bus and do the work at home.... but if i am on a long weekend ride, the last thing i want is to spend time on flats.
While at the shop my wife tried to carry the Curve D3 and the Mu P8 and i have to mention she managed to carry the Mu WAY better than the Curve, first it is slightly lighter and second it has a shorter frame and the rear hub pulling down the rear wheel... (gravity, go figure).... she had to use some power in order to keep it close to her hip.... now this was the D3 and the SL maybe a totally different story.