+1 to all these guys giving good advice.
It usually starts with the budget constraints making you rationalize about getting the f4 instead, but as with others, I had the F4, but now am a happy owner of the f2.8 mk.II. It just makes some pictures out out alive, it is almost magical... I cannot explain it.... the word "dreamy" comes to mind perhaps...
You should not be comparing the 2 lenses based on cold hard facts of F4 vs F2.8 etc ... the f2.8 mk.II has something special... it cannot be quantified... It is heavy yes... but it should not matter unless you have health issues... you will build the required muscles quickly
Wait for Dec 6th? and if you are lucky, Canon announces the 24-70 f2.8 IS or mk.II without IS... if not the 24-105 f4 IS is a great lens for wider shots..