I own the Sigma DP1 and have come to realize that it is not designed to be a rapid fire camera in any way. However the landscape photos I have taken reveal a lot of detail and are generally quite satisfying to me when it is used according to its strength. If you plan to use it for street photography (not something I do at all) it could prove troublesome. I do understand that the DP2 versions might be a tad faster but I have no experience with them. If I was in your position the GXR would be my choice.
Thanks for your replay / opinion !