I strongly recommend that you don't make a purchase until your baby is born. It's great that you're getting ideas now, and it's wonderful to be informed, but you can't make your final decision until you know what your needs are. If your baby is premature, has a really bad latch, or something like that, you'll need to rent a hospital grade pump. Nothing else will get your milk to come in and keep your supply up in the early weeks.
If you're planning on returning to work, you can either rent a pump or use a Medela Pump In Style. I think the Ameda pumps are supposed to be okay, but I don't know anyone who has one and loves it. If you're just going to pump occassionally, the Avent Isis is the preferred hand pump. I've only used a hospital pump and PIS, I know lots of people who like the Isis. In fact, I'm a breastfeeding counselor, and I recently worked with a mom with a preemie who can only let down for the Isis but not the hospital pump. She said that the part you put on your skin is much more comfortable on the Isis.