I smoked on my 18″ kettle for years. It was my first smoker. Slow and sear works well. Snake method also works great and you don’t need to buy an accessory. If you ever needed to use the entire grate for food, you can rig up diffuser plate between the food grate and the charcoal, with either holes or air gap around the outside. If you search Weber kettle diffuser plate, you will see what I mean.
One benefit to a WSM is the 2nd grate. You can actually grill on a WSM if you take off the top 2 sections and just put a food grate on the bottom chamber over the coal basket. So really, either unit can do both jobs. Keep in mind the Weber 26″ has just over 500 square inches of cooking space. The 18″ WSM has like 460-480. The 22″ WSM has over 700. For the price, the Kettle 26″ has more space than even the 2 racks on a 18″ WSM, both being around $400. Also the larger diameter of the Kettle 26″ may be beneficial in certain situations… Large brisket, etc.