Key clubs are usually sponsored by local Kiwanis Club chapters, while Interact is often sponsored by local Rotary Clubs. Both of them are nationwide service organizations that do a lot of excellent work in the community. In my experience, Key Club is more involved in food/clothing drives and that sort of thing (because Kiwanis' big thing is Project Linus, making/collecting blankets for children in need), while Interact does more with organizations like Habitat for Humanity and environmental cleanup efforts.
Either one of them would look great on a college application and help you get the community service hours you might need to graduate (I dunno if your school requires it). I would not advise you to do both, however; talk to the sponsors of each, or attend the first meeting of each, and decide that way.