I have had many bettas over the past 10 years and each one has been chosen for different reasons. Mostly for their unusual colors or combo of colors, both male and female. I also look for one that respond to me. I put my finger up to the bowls and if they come to me, I figure that they are friendly and responsive. I stay away from ones that swim radically or just sit there. All my bettas follow me from my desk to the kitchen (they are on a divider wall so they can see me from both sides. : )
I rescued one very small baby fan tail male that the local pet shop had left in the shipping bag. Who knows how long he was there without food. He was in very little water and behind all the bowls of other fish (don't know how he got there?) He was beautiful multi-colors of blue (dark & powder blue - beautiful !). When I brought him to the attention of the owner, he was shocked. I said I wanted him and how much of a "discount" will you give me since who knows how much damage he might have. Well I got him for $2.99 !! I had him for about 2 years before he died.