Perhaps the reason why I look at Arthur as my cat and Beowulf as Liz's cat is that, when we first got the boys, Arthur was the underdog. He was smaller than his brother and Beowulf used to pick on him a lot. People who would come over -- my mother for example -- would say that Beowulf was the handsomer one. As someone who has always been the geek or the unpopular kid, I identified with Arthur.
The other reason I bonded more closely with Arthur is that, at least at first, he was much more willing to sit still and be petted. Beowulf has a much shorter attention span. Arthur was also the first to flip over and offer me his belly to rub, a sign of respect and submission.
For example, a couple of weekends ago, I was home alone with the boys and they were sitting at opposite sides of the livingroom. I'd go pet Arthur and Beowulf would meow at me for attention. So I'd step over to pet Beowulf and Arthur would give me a look as if to say "what are you doing over there?" So I'd step back over to pet Arthur and Beowulf would meow again. I ended up give each one stroke and then moving back to the other.
Other times, Beowulf will be sitting on my lap or visiting me on my desk and Arthur will come and drive him away. I feel like I should probably intervene to stop this kind of behavior, but I also don't want to harm my relationship with Arthur. It's not like I never spend time with Beowulf. I put in an extra effort to give him attention too (and Liz gives Beowulf all the more attention), but I do feel a little guilty.