Just some followup - I went ahead and bought the 6D + 24-105L kit, and I'm really impressed with both. The distortion at 24mm is pretty extreme but Lightroom's lens profile and the 6D corrections for in-camera Jpegs are quite effective so I can live with that. Background blur isn't as easy to achieve as I'd hoped (not bad at 105/f4 up close, but not awesome), but I have a 85/1.8 and that's wonderful on this camera for blurring the background for portraits. The 24-105L corners are great compared to my 17-40L even on crop so that's a welcome change. (On the other hand the 17-40 corners on FF are really bad at 17mm - nothing like the 10-22 at 10mm on the 50D).
I'll definitely keep the 24-105 for versatility and plan to add a fast 24 or 35 down the road. Thanks for all the advice.