I would go with a 26'er any day of the week over a WSM. I have maintained for years how much better a 26'er is for smoking rather than a WSM. I had 4 WSM's (22.5,18.5,14.5 and the 1880) and I sold all of them except for the 1880. The WSM are good smokers, but they have several drawbacks: First, no wheels. If you want to move it, you can't. Where ever you place it when you start cooking is where it's going to stay. If a storm comes up in the middle of cooking, your screwed and your WSM could blow over if the wind is strong enough during that storm.
Second, three vents to control instead of one. Personally, I don't like to have to keep running around in a circle adjusting different vents just to find that perfect combination sweet spot. A 26'er has one lever to control all the air flow. Third, the vents are down near the ground. I don't like getting down on the ground just to adjust the vents. The vents have no protective rubber grip like the lid vent of a 26'er, so you will have to use your foot or tap it with the side of a spatula to adjust the vents.
Fourth, a water pan to contend with. I don't like to use a water pan. You have to worry about the pan going dry or the water splashing down on the hot charcoal when putting it in or taking it out. I would rather mop my food as I go for needed moisture. Finally, a WSM can only be used for one thing: Smoking! A 26'er can be used for both.
My vote is for a 26'er, no question about it. The only thing that beats a 26'er is a Ranch Kettle ...and I have one of them too.