The main drawbacks to the 6D besides cost, are that it won't be able to mount any of your EF-S lenses. If you don't have any EF-S lenses (or crop sensor lenses), then the difference will be that your current lenses will effectively be "wider" on the 6D than on your xti.
The autofocus will be at least as good, and the low light capabilities will be a LOT better than the rebel xti.
The image quality and low light performance is really great. I have no complaints about the auto focus (unlike several people who don't own the camera). That said, I have tried it at sporting events where tracking is key.
The 6D is very solid in each of those areas. It will cost more (duh), be heavier but more durable. One downside of full frame is narrower depth of field. Also, a 200mm lens has 1/3 less reach compared to the same lens on a 1.5x size sensor. Image quality will be much improved particularly in shadow areas, and if you crop an image, you have a few more pixels to work with.
It sounds like you're pretty heavily invested in Canon's EF lenses already. No reason that the 6D wouldn't be much better than the XTi on all fronts. However, if you can spare the extra $700 or so and wait for a great 5DIII deal, I would definitely go with the 5D. The 5D's AF system trounces the 6D's.