I tend to pick based on a combination of favoritism, genetics, & whoever has the better name. Granted, I'm only on the rose generation right now, so I haven't had to make a lot of decisions regarding picking my heirs out, but for my rose gen I ended up going with my first born, Rizzo. I knew when he was born that he'd be my heir since he had such an amazing connection with his mom that I knew I'd miss him if I picked his brother Pomme Granate or his sister Sangria. Don't get me wrong, I loved his siblings, but his sister was an absolute nightmare as a toddler/child & Pomme just didn't feel like he'd be capable of being a serial romantic. And then obviously Rizzo's daughter is going to be my next heir since he's only allowed one child.