My love to have pentax render images is to strong to stup using pentax. I hope they will come with something better then the k7 for low light work.. The k7 was the only reason I went with the canon 5d mark2.
IF you have to choose now and don't have the time to wait to see what the k5 will offer. I would have to suggest to either go with 5d mk2 or d700 from nikon. The image quality from both cameras are super, and learning a new system is really no effort as long as one knows the basic.
Hope this helps