If you're happy with smoking on a kettle vs. the WSM I'm thinking that you're not really taking advantage of the WSM. The WSM is designed from the ground up for long slow cooks and does that with aplomb. I won't argue that you cannot smoke on a kettle, but it will be more work and results are not likely to be as good. The WSM has the space for a water pan that - even if empty - prevents direct heating of the meat form hot coals. Maybe some of the inserts can do that. Also the WSM can hold a couple decent size butts on each grate. There is not space to do the same in a kettle.
I suggest picking up a couple pork butts (or even 4) and smoking them low and slow on the WSM with a decent rub and some hickory, maple and apple and then decide if you don't really need the WSM.
Edit: I just noticed that you have a UDS. There is probably some overlap between the UDS and the WSM. I suppose for the kind of smokes where the WSM really shines, the UDS is your choice. I have no direct experience with a UDS but suppose it can be configured to perform as well as the WSM.