I have the same dilemma. I purchased a 24-105L last summer and just added a 24-70 2.8 II a month ago.
The 24-70 is an awesome lens! Extremely sharp with little distortion and smooth bokeh. I use it frequently. However, I still find plenty of uses for the 24-105. I must have a good copy as it's pretty sharp above 24mm. The 24-105 is an ideal lens for attending festivals or day outings when I don't want to carry multiple lenses. It's nice to have the 70-105mm range when carrying a single lens and the IS saves shots that I would probably blur otherwise.
So, I'm hanging onto my 24-105 for now. If it starts to gather dust at some point down the road, I'll sell it.
CANONisOK: $800 is a really good price, I would take it since you haven't been using the lens anyway.