Morgenhund has it right. There is no one answer. You need to do more research to determine what's right for you and your business.
I'd suggest you research your competitors and find out what they're using. Then see if you can learn how long they've been using those resources. The assumption I'm making is that if they've been using them for awhile, then it's likely been working well for them.
ad words. no question about it. shopzilla and bizrate are the same company. shopping is also a CSE. the problem with CSEs is that, in most cases, they're getting their users from ad words. you'll get better ROI going straight to the source.