I admit that I'm a Walker homer but I have 3000 hours on mine so I do speak from experience. If you are going to use it mainly for bagging it is definitley the cadilac of mowers. As far as it clogging up, it will if you are bagging tall, thick, extremely wet grass. Probably five times a year the dew is bad enough to clog it up, but I always schedule my bagging yards for first thing in the morning to take advantage of the Walkers ability to bag. If the grass is fairly dry no matter how thick it is it will never clog. That being said, I would never buy a Walker to side discharge or mulch with. It is'nt the most comfortable mower or the fastest.
I bought the 21 hp Kabota and haven't touched it 3000 hours. I don't think Walker offers this option anymore so my next one will be a diesel. If you do buy an air cooled, make darn sure you don't let grass or leaves build up around the engine as it can cause a fire.