Either of your cleat sizes will work so long as the bench is fitted well enough to the niche that the bench actually sits on the cleat. Obviously, a thicker cleat is more forgiving if you're sloppy when fitting the bench to the walls. I'd screw the cleats tightly to the studs and cabinet sides, probably with #10 or larger screws.

It will take very little to fasten the bench to the cleats, since all you need to do is keep it from slipping sideways. Almost anything will work for that.

Thanks Jon for your reply! That answers all of my questions - I'm ready to go forward with the project!

It occurred to me after I wrote this that while such cleats should be able to support a great deal of weight, I wouldn't feel great about them if impact were going to be an issue, such as it would be if you had a gyrating, ipod-driven teenager who was prone to jump on things. Good luck with the project.