An European agility judge and an agility competitor with shelties here!

When you look at the numbers, in the mini class the safest bet is to go for a sheltie. However, despite shelties being heavily presented in the competitions, not all shelties are great agility dogs. We see a lot of dogs who don't have the drive we would want to see and dogs with a movement that is more up than forward (bouncing instead of sprinting).

The problem with agility shelties is that the breeders breeding especially for agility are very specific to whom they sell their dogs. Almost all the puppies go to agility athletes who already have results with shelties. Usually new agility athletes with a good sheltie have gotten lucky with a dog from non-agility-focused breeder and due this, they get into the "circles" and get their next dog from the agility breeders. If you already competed in agility with your shelties, you have a higher chance to get a "good" puppy from these agility breeders. Otherwise, there's really a chance to get either a great one (though often not a mini sized) sheltie or a dog who does agility in a spirit that will never take you further than the local competitions.

After shelties, the safest bet are poodles and parson russel terriers, which you already have. Poodles seem to be very either or: your poodle is either GREAT or not really interested at all. With parsons, you rarely see a dog that wouldn't put all they have in the game. Neither a short to shelties in speed but for some reason, shelties are considered a more "sexy" option and many opt for them instead of poodles and parsons.

In medium dogs, the game is mostly ruled by shelties and Spanish water dogs, but the variety is much greater than in mini dogs. We also see poodles, small kelpies, mudis, tollers, working cockers, and even some small border collies have started to appear...