Crash pads are designed to cover your elbows and knees. Take time when deciding which set to go for so that you've got a good fit that won't slip or be knocked out of position (espec. the knees).
The pad bindings (which are usually elasticated velcro straps) need to be comfortable and not chafe as you're going to spend a lot of time wearing them.
Another consideration is to see how easy it is to bend right down, particularly if you're in a buggy where your legs maybe bent for long periods of time. Generally speaking you get what you pay for, spend more money, you get more comfortable pads.