6-inch cans are for Br40s, and 5-inch are for BR30s. The last owners of my house put BR30s in the 6-inch cans, and not only did they look lost inside the giant cans, they also were too dim.

I've been replacing the Br30s in my basement (Juno cans) with Feit/Lowes Br40s (17 and 22 watt versions) and they are MUCH brighter (and much brighter than the CREEs). And when they do burn out, I won't have to worry about having mismatching trim (will CREE stil be making CR6s 20 years from now?).

The only 6-inch cans I've had problems with are some of the shallower one that are often used in insulated ceiling (e.g. Halo H7icat). For those I've used the new Lighting Science/Home Depot Ecosmart BR40s. They have the width of a BR40, but the depth of a Br30, so they don't stick out of the fixture.

http://www.homedepot.com/p/EcoSmart-15-Watt-90W-BR40-Soft-White-2700K-LED-Light-Bulb-ECS-BR40-90WE-W27-120/203914616#.Uoom3SiQ4wE