It depends, really. I've gotten a few really nice Bettas from PetSmart, but I've also gotten some amazing looking ones from PetCo..
PetCo does sell higher end Bettas, they seem to get them from great breeders and PetSmart is now breeding their own fish, including their Bettas and they honestly don't seem to know as much about the different types. I often find PetSmart to have mixes of HM and DT labeled as HMs. Since PetSmart is breeding their own you can often find quite young ones there too, under a year but not in their fry stage any more, unlike PetCo who usually has either fry or 1 year+.
My PetSmart gets their Bettas in on I believe Friday or Thursday, while PetCo gets theirs in Wednesday. You may want to call your nearest PetCo before diving out and see what day they get their shipment of Bettas in. You don't want to drive out and have them only have about 4 VTs left or something.
Also, it varies between PetCo to PetCo, my local store has about 8 different types of males while the one in a neighboring town has double that as well as higher end Bettas in color and posture.
Here is a handful of photos of Bettas I have gotten from each store:
Petsmart boy (got real lucky with him! they never turn out this great):
(img may stretch screen, so i left with just a link)
PetSmart (Delta labeled as a HM):
PetSmart (i love this guy so much lol, he's grown the most):
Now none of these guys are super amazingly amazing like PetCo can get in, but they are uhm, yes, "higher quality" Bettas and they keep getting better and better ones in. They are also much more expensive, so remember to bring your money with you, you can end up buying a $24 Betta. But you never know what you might find, or who may find you. Currently I only have 4 Bettas, two are from PetCo and two are from PetSmart.