k5 iis low light af speed
After many years with Pentax, the 6D is the first Canon body (outside the top-end 1 series, which are heavy and expensive) that I really like shooting with. I never warmed to the 5D or 5D2 at all or the XXd series, for that matter.
I bought the 6D last fall (refurb from Canon for $1,200) for a trip to Antarctica to go with the 7D I had bought previously in order to gain access to longer lenses than Pentax offers. They make a nice combination. I mention all this to make it clear I didn't buy the 6D because of any special enchantment with FF. I bought it so I could bring a camera with image quality on a par with my Pentax gear, which I wasn't going to take with me on account of weight restrictions. The 7D image quality is rough at higher ISOs.
The 6D's image quality is about equal to or slightly better than that of the K-5IIs; its advantage is mainly at higher ISOs. I have occasionally found I get better WB from the Pentax, but since I shoot RAW that's not a big issue.
You can have fun shooting with limited depth of field on the 6D. I don't own any f/1.4 Canon glass, but might break down and treat myself to the legendary 135/2 lens they offer so I can do the dreamy look now and then.
Autofocus is quicker and more certain than anything I've shot from Pentax, though I haven't touched a K-3. It's not up there with the top Canon bodies; my old 1Ds is faster. Haven't really paid attention to the 6D's write speed, though it may be better. Note the frames per second is not blazing fast. It's a portrait/landscape camera, not a bird/sports specialist.
In general I like the 6D quite a lot. It's a different animal from Pentax: more menu-driven, more plastic in the body, less solid feeling. But it's a very capable, unobtrusive camera (nice quiet shutter) and pretty natural to use -- and it's awfully nice to have autofocus that really works.