Don't quote me on this since I'm not 100% certain, but I believe the main difference between the two is that kegel balls are of a ball-within-a-ball design so that the inner ball rolling about stimulates the PC muscles as the wearer walks and moves, while ben-wa balls are just stand-alones that you grip inside you to achieve the same results. Kegel balls are usually attached together in some way, and have a pull-cord for easy removal.
The idea of both is the same: they aid in kegel exercises (though strictly speaking these exercises can be done on their own, without any tools), which tone the PC muscles, which in turn can have a number of helpful effects. Strong PC muscles have a good grip, which can make sex more pleasurable for both parties, and can even help the woman have stronger orgasms. Women concerned with looseness, like say after childbirth, use them to help return to their original condition. Women with incontinence problems can also benefit from kegel exercises.
As materials go, it's the same as with anything else you may put inside you: silicone, metal, and glass are the safest and easiest to clean, and plastic is also pretty good - you just can't boil it. Size basically depends on what you're comfortable with. I don't abstain (my actual sex life is dead at the moment, but I've got plenty of dildos, hahaha!) but my plumbing is also on the smaller side, so I know how you feel. I use the Luna Beads mini version
myself, though I've also used the Emigi
and regular Luna Beads
as well. The former was alright (though the cord was a bit annoying and I couldn't really feel it working like I can with the Luna Beads) while the latter could, at times, be uncomfortable to insert or remove - mainly if I hadn't toyed in a while.