My kettle (Performer) is the go to for every night grilling, but my WSM is what I use for smoking, low and slow. I just feel like I have better control of the fire in the smoker.
I love pecan with chicken or turkey, but a a pork butt can really use some hickory. You might try both. I usually start with three or four chunks, and add some more when they are gone. A butt can take a lot of smoke IMHO.