Oh, what a pain in the ass gear can be. So I have an XSI and I love the output, but the screen sucks for checking focus, buffering is quite slow (shooting RAW + JPG) and focus is hit or miss.
I typically only take pictures of my twin girls who while only 10 months old move enough that nailing focus on the XSI requires burst shooting to hopefully get a sharp picture. Checking focus on the XSI lcd (which I do quite a bit) is difficult (the lcd color/contrast is poor and the detail always looks soft). I also shoot indoors quite a bit and prefer natural light to flash whenever possible and XSI images fall apart above ISO 400.
I want the very best picture quality and I want a camera that can handle the focusing performance requirement of moving kids ( I shoot candids of people as well).
I have pondered the 5D Mark II and as a former Nikon shooter I have also looked at the D700 considering both price points are fine. Staying crop I have also looked at the Nikon D7000 and the Canon 7D.
I would move back to Nikon, but my biggest concern is Nikon colors which I feel always looked a bit artificial (I just love Canon skin tones), on the other hand I like the dynamic range of Nikon's sensor which seems to be lacking on Canon sensors.
While I do shoot JPEG + RAW, I find myself using the XSI JPEGs 99% of the time because out of the camera they are a close match for the RAW files so I do perfer a body which can deliver a solid OOC JPEG.