I'm a little confused by your post (OP) as you say you PREFER some...is that not just another way of saying one is, in some way, better than another....that's what a preference is.
I do not treat any of my pets with any more or less care than any others. All of them are nurtured and tended with equal consideration and I love every one of them to bits but yes, my cats have a greater hold on my heart, then my buns, then my other animals....hoggies and chooks.
It is only natural that you bond with some more than others and that creates preference or favouritism. My cats are indoor cats. They are there every minute I am at home. If I am sat down, they are in my lap. They lay on my shoulders while I do housework, sleep on my bed, greet me when I come home, they are more precious to me than I can possibly say. I am still mourning for my beautiful Max who I had to have PTS last February. I still cry regularly and miss him so much.
My bunnies are outdoors. I sit in the pen with them in good weather, they run round me, sniff and snuggle, eat from my hand, are thoroughly adorable. I love them to bits. I think of Toby and Winter, my previous buns who died. I am sad they are no longer here but stopped crying a long time ago.
My chooks....I spend time with them every day for about 30 mins before work, love to watch them, hand feed them, one or two even sit on me for a pet. I love them too but have had to cull in the past, when they've been poorly. I have cried as I did the job and mourned their passing for a short while.
I could never bring myself to 'cull' my cats or rabbit if they were too poorly. I could not take their lives myself, no matter what. It would be too painful for me to do it myself, I would have to take them to a vet. The principle behind it is no different, the chooks get an instant death, painless and humane and there is enough emotional detachment to make it possible to do. I could just as easily PHYSICALLY cull the cats or rabbits but my heart would never allow it.
This is not because I don't care about my chickens but because, no matter how you look at it, some animals are lower ranking in emotional attachment than others....it is inevitable.