In the end, I decided on the F2.8... In recent months, I've used the 24-105 F4L much more than the 24-70 2.8L because of that extra telephoto reach, and I have to say it works very well except at sunset and indoors, where I struggled to take good photos without too much grain.
At those times, I missed the 24-70 F2.8L. That's why I ruled out the 70-200 F4 and broke the piggy bank to buy the F2.8. 🙂
Some more photos with the 24-105mm F4L