Hi Catsox. That's an unusual and interesting situation to be in. You could make an argument for all three possible combinations of 2b's here...
Staying with Boone/Durham is probably the safest way to go. Butler hasn't been up long enough to prove that he's for real, although the early signs do look encouraging (plus, he's been considered an intriguing prospect for some time, going back to his days in the Cards organization). You know what you're getting with Durham and Boone, and can also hope for some improvement. I'm not terribly worried about Boone's bruised knee, so that shouldn't be a factor. Besides, these two are an excellent combination, since you can choose whether you need more speed or more power at any given time.
If you want to take a chance, however, Butler would be an excellent choice. I think he'll be pretty successful, and playing half his games at Coors definitely won't hurt. If you have daily lineup changes, you can also go with a Colorado platoon, playing Butler at home, your other 2b the rest of the time.
If the choice were mine, I'd grab Butler. I expect Boone to improve on his first half numbers, but not enough to justify passing on this opportunity.