Have you ridden either system before (LOOK vs Shimano)? I've been riding LOOK for years and am currently using a CX6. I love it. I have, however, never ridden the Shimano system.
Downside to a LOOK system is cleat wear when/if you need to walk on them, as well as moderate difficulty doing so. Happens so rarely for me that its not an issue at all in my experience. Shimano supposedly makes this a bit easier/better. I have also heard that those who go from LOOK to Shimano at first feel the Shimano to be a bit looser floating-wise, but that it then becomes preferred. This here would be preference, so maybe see if you can test ride each at a LBS?
Good pedal systems either way.