A bit over 1 year ago, I bought Atomic Blades to fit over basic 5mm boots. I have narrow feet and generally wear size 10.5 shoes. sometimes 10, sometimes 11. My LDS sold me size XL Blades.
I finally decided I thought they were too big and picked up a new pair in size L on Closeout. They fit me perfectly! My XL Blades will be going on CL as soon as I can get around to taking pictures and posting them.
The XL foot pocket is a little longer than the L, but that didn't cause me any problems or discomfort. it just meant the straps went almost-but-not-quite straight across the back of the fin when I was wearing them. When I anted up for Atomic spring straps, they were too loose, out of the box, but they are made to be able to be adjusted (3 total lengths) and when I set them to their middle length, they hold the fins on just fine.
The biggest difference I actually notice is that the L foot pocket is noticeably narrower and feels just perfectly snug on my booted foot, where the XL had a small gap on each side of my foot. With the L's, I put the spring straps back to the longest length and they work fine.
I'd say if you're planning to wear thick rock boots, you might want XL. Otherwise, you probably will like L better.