As you stated, obviously the overall height is the main difference. Then there's components that change because of that (coils, rear blocks, shocks, transfer case indexing ring, etc.). Really the decision comes down to how high you really want the truck to be and what wheel/tire setup you plan to run. A 6" will ride slightly smoother because the long arms will be more horizontal than an 8" however I don't think that will be noticeable enough to factor that into your decision.
I would upgrade the shocks from the BDS 5500's that come with it no questions as they're just a generic white body shock. I've been happy with the Fox 2.0 shocks that came with my BDS lift. I would definitely recommend a dual steering stabilizer setup as well. Again, I run the Fox 2.0 shocks and don't have any complaints. My truck handles great and drives straight as could be down the highway without even touching the steering wheel. You don't really need anything else from BDS however an adjustable track bar wouldn't be a bad idea and if you're doing the 6" kit, a transfer case indexing ring wouldn't hurt either.
As far as other extras go, I would get a steering box brace if you don't already have one.