There is a difference in cooking with either one, but, done properly and with experience, you can turn out excellent product with either cooker. As a person who owns a kettle and a UDS, I would gladly add a WSM and a PBC to my cooking tools. Each one cooks a little differently, allowing for more flexibility.
I think the process differences are quite clear, once you understand each cooker. I totally disagree with people who say you have to pay lots of attention to a kettle and little to a WSM. I can setup and run my kettle for 4 to 6 hours with no intervention. I would do the same with a WSM. The end product is also hard to distinguish, I can produce great product off of either one. But, the WSM footprint is MUCH smaller, relative to amount of food you can produce.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."