My limited experience with lump is that it burns a bit hotter, and not as consistent at briquets. However, I'm sure there are people here that use only lump in their drums. They can help you if that is your preference. However, I think overall briquets are more forgiving with temperature control.
For the diffuser, you can use a pizza pan, a terra cotta saucer, an empty roasting pan, or even a grill covered with foil.
I use a perforated 16" pizza pan I picked up at target. I set it right on top of my charcoal basket. I have NO hotspots, and my fuel efficiency is not affected.
-elbows deep in someone else's misery.