Here's what bugs me about bonding: No one ever defines it. Since no one says what it is, you can never know if it's happened, or happened correctly, and it can be a source of infinite anxiety. My temptation, therefore, is to tell you not to worry about bonding.
I'm being all about me here, in the hope of providing supportive and useful examples.
I didn't have an overwhelming rush of emotion after the birth of either of my children. In fact, the first thought through my head, both times, was "what was I thinking?" I generally attribute this to blood loss. I was not in good shape. On top of that, I have never fallen in love with anyone at first sight. I have to prowl around and get to know people before my emotions get really involved, which meant that I did spend a while when both kids were small going through the motions of just caring for infants as well as I knew how while not really grooving on it. One day when my son was about three weeks old, I spent a while playing stupid baby games with him - doing sit ups while he leaned on my knees, and pretending to find him every time I came up - and after that I felt like we were buds. DD was a tougher case - she was in the NICU for over a month, and I had PPD, and there was sleep deprivation - but after I'd had a few nights of good sleep, I looked at her one day and realized what an immensely charming baby she was, and then we were all good. There will be a moment when you begin to feel connected to your baby. It's okay if that moment is not at birth.
It's been my experience that overwhelming emotions are transient. It takes a lot of energy to sustain them. In order to be deep and abiding, an emotion has to be felt on a level that I have the energy to both feel and express every day. Now and then, I get hit by waves of overwhelming feelings about my children, but those waves are brief - half an hour at most. The feelings that get us through our days are quieter, they leave room to multi-task. I have to be able to love the baby AND wash the dishes. Or: I have to be able to love the grade-schooler AND keep the pre-schooler from repainting the bathroom with toothpaste.
I have frequently felt that I like one child more than the other. It breaks down this way: My favorite child is the child who is currently asleep. If they are both either asleep or awake, I favor the child who is causing the least trouble. When all other things are equal, however (both children asleep after a peaceful day), it would kill me to have to choose between them.
You love the children you have, and you will love the new baby, too. Don't worry about having exactly the right feelings, and especially don't worry about having exactly the right feelings on deadline.