I literally had the same question. I pulled the trigger on the 100L during the previous sale posted on CR. Unfortunately, they sold out at 50, but it was $870 pre-rebate for a New model from an authorized dealer thus essentially $720 post rebate. Holy cow, what a wonderful deal, especially since I've seen them no less than $850 used in my local craigslist ad, (and I try to avoid CL like the plague).
From the research I've done, they both make sense, however, the L is still the dominant lens (price aside), therefore, and an investment, I waited until the L fell into my budget and whammo!, it's here next week.
In addition to the beautiful price checker found on this site, canonpricewatch has also been a friend of mine in the past and I support both of their efforts.
I hope this helps! Good luck to you!