First off, I use different sizes for different exercises. I don't want to have to mess with .changing out the kb itself in between exercises.
Also from what I've seen, the one with plates only goes up to I think 25 lb. So it's maximum weight is my minimum weight and used only for overhead stuff.
The ones that you add sand/shot to can be made heavier but you've still got the hassle of changing it out.
I don't see the benefit of adjustable bells at all myself. :o)
Swing Like You Mean It- RKC Style
Currently swinging either a single 55 lb. or two 25 lb'ers.