They are all very different, and I will only talk about WSM and offsets as I have lots of experience smoking with both of those, and much less with a kettle with those sort of mods.
A WSM is a great cooker. It produces consistent results and is a very easy to use cooker. It uses tremendously less fuel than an offset, and can run anywhere from 8 to over 24 hours without needing to refuel (based on charcoal used and other factors). WSM's are what I use. I find them to be the best world-class cooker value-wise. (UDS can be argued here but you have to build your own).
An offset is a good cooker as well, but I would really only look into one over a WSM if you are talking about a larger offset, one that can burn split logs as the source of fuel. The flavor from a pure stickburner is the best in my opinion, but it comes with a cost of labor to keep it running. They usually have to be refueled every couple hours or so, and you should keep an eye on them since you may make adjustments throughout your cook, so it requires a lot of attention compared to a WSM.
People have won championships with both WSM's and offsets, so the flavor difference is not that dramatic or important, but it is there (just my opinion, others may disagree). If you have the time and the money to run an offset, that would be my primary choice, and after that I would never hesitate to go straight to a WSM.
You may also consider vertical units like Spicewine, Backwoods and Stumps cookers if you are considering the WSM route, but have the money for a large offset. Just fgood for thought.