I bought my 24-70 2.8 L mk1 last February when the mk2 lens was announced, before the demand and prices shot up on the mk1. I was very lucky, as I think I have a very good copy, and really love it's overall performance.
After using it awhile however, it became clear that it's a very heavy lens, and not the best for walking around with it in hand or around the neck all day. I decided that I wanted a high IQ midrange zoom for general use, but not one that's so heavy, so I picked up a 24-105 4 L IS. It was a great choice. It's very good with out being so heavy. In fact, I think the two lenes compliment each other very well. The 24-105 works as a walkaround lens outside where the light is usually fine, and the 24-70 shines for indoor/low-light and great bokeh.
My advise is to go with the 24-105 kit lens, as you will get a sweet deal with the 6D body, and its an excellent general purpose walkaround lens. If you find a good deal on a 24-70 mk1 or mk2, you can add it to your collection later.