Think about what you want it for. If you are going to be expressing at work for a youngish bub then speed is of the essence, so that you actually get to do something other than express in your breaks. In this situation a double pump is very useful - medela and ameda both make good ones. Similarly if you are mostly expressing then get a double pump and claim back a couple of hours in your day.

If you want to express a few times a week for the freezer or to have a night out or when it doesn't matter how long expressing takes you then a single pump is fine. The medela mini-electric is a light-duty beastie, and is a bit heavier to hold than other pumps because the motor unit is on the bit that you hold onto your breast.
Other more expensive pumps have a hose to the motor unit, so you put the motor unit on a table or the floor and just hold the bottle and snozzle parts to your breast.