I bought a 26.75" kettle last summer and have been very happy with it. It's has a lot of real estate and is very versatile - I've used it to grill and to smoke. As far as grilling goes, it can hold a lot of food and can do high heat cooks. Smoking is relatively easy too, but it can be a pain to add fuel every so often. That said, I've smoked on it more than I have grilled, so I recently bought an 18" WSM, which I assembled last weekend. I haven't smoked on it yet, but I look forward to being able to smoke on more of a set and forget basis and being able to smoke more/different types of food at the same time.
To address your question, would you like to smoke or grill? The 26.75" can do both, but really excels at grilling. Having not used the WSM yet, I imagine the converse is true.