All good advice, I would add that sometimes a quiet composed dog attitude is better than the hyper active type. In my training group,my Lab is fast and excitable, but not so accurate as a little terrier who seems not very interested, but has the best contacts and makes fewer mistakes! We have just started competing, and it's all about clear rounds. The terrier looks the most likely out of about ten dogs.
I'm pretty sure that all dogs will enjoy agility, and the sociablity is very good as well.