What qualities do you look for when choosing e-commerce shopping cart software for a website? I don't know how you go about picking one.

Are the features common to all? I know some are free and some are paid. I would worry that a free one was less secure than one I'd paid for. Is there any difference in the level of security or are they all similar in that respect?

Please share your thoughts on what is the best e-commerce shopping cart to go for and why. If there is anything that I'd definitely not want to use I'd like to hear about that too. Thanks!