Hey Holly. Are you planning on expressing for the purpose of donating to milk banks or would you like to be able to feed your bub expressed milk? Or is it just to get yourself comfortable due to an oversupply?
If your worry is oversupply then you'd probably be best not to pump too much at all, particularly in the first few weeks. In which case, I'd go with a manual pump (if any - you could always hand-express excess milk). Expressing a lot means you're going to make heaps more, so the advice I've always received from ABA folk is not to express too much, particularly in the first 6 weeks, if you want to regulate your supply to your baby's demands.
If you're planning on feeding bubs expressed milk or doing a lot of donating (not sure that there are any milk banks around here?) then my personal opinion (FWIW!) would be to go for an electric. I've not used them before but they've got to be a billion times less boring than expressing a bottle-full from a manual pump. Ugh, soooooooo booooooooooring! LOL!
GL and if you'd like a 2nd hand manual pump for nuthin, look no further.