Personally, I have found that the best smoking machine - bar none - is the Weber 26.75 Kettle. I used to have the big 22 WSM, the 18.5 WSM, and the 14.5 WSM. To me, the drawbacks to the WSM have always been four big things: Nothing to hold the lid for you when you want to tend to the food, no wheels so it has to stay wherever you put it until you're done, the water pan (I don't like using them), and having to control 3 vents. I found that using the 26.75 Weber kettle solved all these problems. It has a lid bale, it has wheels so it can be moved while cooking which is really handy if it starts raining in the middle of a cooking session, no water pan to fool with, and best of all temperature can be controlled with one single vent instead of three.
In addition to all these benefits, the 26.75 kettle has a rubber tip handle on the lid vent so you won't burn your hand, it has a one touch lever underneath so you won't burn your hands there either, it has an ash collection bucket underneath so you can clear ash away as the charcoal burns throughout the cook, the food and fire can both be tended to at the same time without having to bend over and it can be used as a grill or a smoker.
To me, I have always thought that food had a much better bbq taste on the 26 vs. the WSM. I guess because the smoke and heat were closer to the food? Also, I have always had better temperature control on a 26 rather than a WSM. The WSM controls temperature good, but the 26 is better. This is because there is only one seal on the whole unit: where the lid and bowl come together. On a WSM you have three seals: the bowl and the barrel, the barrel and the lid, and the access door against the barrel. Also, with the arrival of the new Slow n' Sear specifically made for a 26 kettle, smoking is even easier.
The WSM is a good piece of equipment, don't get me wrong; but if I had to choose either a 26 kettle or a WSM, I would choose the 26 without even thinking twice. If I had to choose one of the WSM's, I would go with the 22.5 model, that way it covers everything you might ever want to cook.