I'm only considering a Corgi because I know I have the time to devote to training and activity. Basically every breeder I've talked to has mentioned that they are not a "couch potato" breed, and if you don't have the time/energy to devote to them, you can end up with a little (sized) big problem. They may get bored with routines and going on the same walks every day (although this can depend on the individual dog). Greyhounds, on the other hand, are often referred to as 40mph couch potatoes. Corgis can keep up on hikes, but if you like going on really long hikes (more than a few miles), they'll probably start to tire out. They can also be quite vocal, as unlike border collies, who herd rather quietly, Corgis use barking as herding tactic. If you look up YouTube videos of both breeds herding, the difference becomes apparent immediately.