There really isn't any difference in the BTE and body worn other than with the body worn, more programs are capable of being downloaded (depending on what brand you are getting). I chose both BTE and body worn, but honestly I never use the body worn. Just sits in a drawer at home gathering dust... and as far as batteries, well the batteries on the body worn may be rechargeable, which is good, but they die out quicker and it can be a pain in the bum to have to carry a recharger around all the time. Batteries in the BTE generally last me 3-4 days and it only takes a couple seconds to change them. By the way, when you get your CI package you will have enough BTE batteries stocked in it to last you more than a year. So you won't even have to think about buying any for a long time.
Some people love their body worns though.. so do ask your audi if you can try out a body worn one as well as a BTE for a couple days... I've nothing against the body worn ones other than that they are cumbersome.