I think Suzanne Clothier says it very well here:

http://www.suzanneclothier.com/the-articles/dances-dogs

I think dogs would choose a class that caters for their natural behaviours. For one of my dogs, he isnt that keen on obedience classes where it's heelwork and stays etc because thats not what him as an individual is wired to do. Yet take him to gundog training, agility or do some tracking with him and he couldnt be more enthusiastic because his 'thing' is retrieving, using his nose and general running and jumping about.
Yet my collie likes obedience and maybe thats because he's much lower energy and obedience doesnt require him to be tearing about at full speed. Plus he's very sociable and enjoys having other dogs around him. My gundog isnt so sociable and prefers to have space between him and other dogs.

I've read that article before .. like alot of her stuff ..particularly the "He just wants to say Hi" article - a must for every dog owner!!

Oh I like that one too!