Have to agree with the others. Certainly a combination of the two, but I believe genetics lays the important foundation that you'll have to work with. My older Sheltie is from conformation lines exclusively and quite honestly, she's a little "slow" in the brains department. No toy drive, no instinct, but she's very pretty!! She makes a wonderful therapy dog and is pointed in conformation so I know she's built properly, but agility is exhausting because of the need to cheer her on the whole time.
When I went looking for a puppy, I went to a performance breeder in Canada who's active in agility, obedience and conformation. The pup I got from her is DRIVE defined and naughty, beautifully built (could have shown in conformation if not 1/4" too tall and his ears were done) but also has a wonderful off switch. I did a lot of impulse control training and chase games etc. with him as a pup (he's almost 2 now) which I'm sure made a difference, but the willingness to please, thinking brain, functional structure and working ability is credit to his breeder. I just encouraged it! And boy, he's going to give me a real run for my money. My one gripe is that he can be noise and environmentally sensitive, but the GOOD news is...if there's agility equipment or sheep in front of him, he could care less about anything else.
I'd rather have just a bit too much drive and have to tone it down with impulse control and clear communication than not enough. So I think it's important to look for a line that has it and then choose the pup based on its personality, intelligence, willingness to engage and structure. Just based on my own experience!