The difference between Search Perks and Cash Back are minimal, but the idea behind the new creation is that you get points for every search. Those accumulate and then you can win prizes.
You're limited to IE6 or higher to participate, so Firefox users are not eligible. That's because SearchPerks has a built in toolbar.
It's questionable, though, if this is a good business plan for Microsoft. To me, it sounds like something that can be easily exploited. One forum member says the following to echo that sentiment:
If people are doing pointless searches and meanwhile clicking on ads to max out their "tickets", I can't see it being a good deal for anyone in the long run.
Many people agree and think that Microsoft should stop while they're ahead. But until then, happy searching!
Forum discussion continues at WebmasterWorld.