Personally, I found the Controller Sheme D to work the best. The default one just felt to unatural for me, I really didn't like how the Right Bumper was crouch and lay down, couldn't get used to that concept...
I've been using the default settings. I kind of like the crouch/prone action on LB - it makes the running slides and dives much easier for me. And you pretty much need those moves if you want to cross one of the bridges in the demo map.
I tried a couple of the southpaw but came back to the default. It took a while but I just had to retrain my brain. What I haven't gotten in the habit of using is that lean button.