I work at Petsmart, but I'll tell you for selection, Petco hands down. Petsmart's (at least mine) are healthy, but they only off male veil and crowntails, and females veiltales, occasionally a femail crown. Petco have literally every variety of betta, ranging from about $5 for female or male veiltails so about $24 for king bettas (in my area, at least). And I agree with above, try to go a day after shipment arrives, you'll get the best selection and the least depression.
I got my halfmoon doubletail there, $15, he's young but he was 100% healthy.