When we chose a betta for my daughter, it was kind of a toss up really. They were all in very cold water so were sluggish. There were some marbles and some reds. I knew that whichever we picked for her would never be bred, so I looked them all over (though not the females as I had decided to get her a male). I chose Hannibal in the end. He looked a bit pathetic as he had most of his dorsal missing. A lovely blue with black head and red on his fins. He's one crazy, messed up fish but interesting to watch.
I've recently bought 6 for myself. I bought them from DarkMoon. How did I choose them? Had my eye on one of the males from the start. I thankfully was able to get him. He looks chrome colored with his dragon scales and so looks like my favorite color. XD The second male I liked the looks of and the design I saw on his dorsal. Third male has red on his fins and something about him just pulled at me. The girls... I picked 2 black dragon females for looks alone. The third is a marble female. Took me a while to pick her out. I knew I wanted a third so I could start a sorority, but didn't want to get one I didn't like. I don't normally go for marbles, but something about her made me want her.