Choosing the right breast pump is a personal decision - and one that can be very confusing when you're a brand new mom and not quite sure what to expect or what you should be looking for. Two of the most important things to consider are how strong the suction is and the number of cycles per minute it has. These two features will determine how much milk you collect and how long it will take.

Determine how many times a minute the pump cycles. Most babies suck between 40 and 60 times a minute when they're nursing. The hospital grade rental pumps have a 50 times per minute cycle and automatically mimic the suck-release-relax sucking pattern of your child. Pumps with smaller motors will generally have a slower cycling rate and may result in sore nipples from the longer periods of sustained suction.
Check the amount of suction the pump applies. A baby will normally have a suction rate of 220 mm. Many hospital grade rental pumps can achieve a suction rate between 200 and 220mm.
Decide if you need a portable pump or if you will only be pumping at home. The hospital-grade pumps you can rent can be too cumbersome to regularly take on the road with you, but they are very efficient at pumping.
Look for an electric pump that has the suck-release-relax cycle that most closely mimics a baby's natural nursing pattern. Some models automatically simulate this release cycle while others will require the mother to use her fingers to simulate this pattern.
Choose an electric pump with adjustable controls to make the pumping action best fit your style. There may be times where your nipples are sore or your breasts are engorged and you will want to be able to dial down the suction to a comfortable level.
Test drive the pump for noise and ease of use. If you're going to be pumping at work or somewhere privacy is important, make sure you turn on the pump and test the noise level while it's in use. Some models are quiet but some with smaller motors can make a surprising amount of noise.

We have personal experience with these breast pumps and recommend them if you're considering pumping:
Single battery/electric pumps
Medela Mini Electric or Medela Single Deluxe automatically cycle between 30-38 cycles per minute, a rate that will provide enough stimulation and will be comfortable.
Occasional-use double pumps
Medela DoubleEase or Medela Double-Deluxe pumps automatically cycle at 30 times per minutes or above.
Simultaneous double pumping professional grade pumps
Any of the Medela Pump In Style breastpumps.
Simultaneous double pumping hospital-grade pumps
Medela Classic, Medela Symphony, Medela Lactina, Ameda Egnell SMB or Ameda Egnell Lact-E.