Be careful and check out your 24-70 II when you get it. I could not get a good one. I could not get them to AF as accurately as my mk I version on two different bodies. I had high hopes but decided to return the last one and keep my original. Now I am exploring primes in that range. Got an 85L which I am infatuated with to a fault. Not exactly in that range but working my way down as my bank acct allows.
Something else to keep in mind though most wouldn't realize...you also tend to get more light transmission with a prime in general than with a zoom. The additional optical elements can give you up to 1 stop less light even at the same f ratio setting. Could be important if you do a lot of low light shooting. One example I can give is that comparing my 100L with my 70-200L mk II at the same f stop and focal length, I lose 2/3rd of a stop worth of light on the zoom.
Might not be important to you but just be aware in your decisions....and test your new lenses every which way and then some to make sure it lives up to your expectations.
Wow, I wasn't expecting so many great responses!
It's true I've only shot with a few primes (the Canon 100L & 200L, and Sigma 35 Art), so my experience is limited in that regards. But like I said, I rarely zoom in and out in between the widest and longest ends of my zooms, so it seems only logical that I could benefit from the lighter weight and faster aperture of primes; granted I'd be losing out on the convenience and versatility of my zooms if I choose an either/or scenario.
So I think what I'll do is purchase the 40 pancake and 85 II for now and see how I cope with not being able to zoom. I actually returned the 24-70 II a few weeks back (forgot to update my sig) due to $ constraints, but I grew very fond of it while I had it. This is why I'm in a position to make a switch now.
If I go with the 24-70 II again or the 24 II, I'd buy it reburb from the Canon store (as I did with my 70-200 and will do with the 40 and 85 as well). Or, for the cost of a 24-70 II, I could buy the Tamron equivalent AND the Sigma 18-35. Basically, I have about $3800 to spend and I'd have that much more if I sold the 70-200 to reconfigure my kit. IDK, all your input has got me really thinking and torn whether this is the best decision, and unfortunately the closest camera store with any of these lenses in stock is over an hour away