Hmm. Sweepstakes would make more sense to me, since lottery just makes me think of the state lottery, where you pick some numbers and wait to see if they draw the same ones. Which is pretty much the same thing as a raffle except raffles make me think of charity drives or small festivals. Maybe it's a size thing?. I'm not sure how they use the word lottery in British English. Maybe lottery is the right word over there. They're all pretty similar. I think contest might work too. We do have online sweepstakes here in theory, but I've never found one that wasn't a scam, so I think I'd be confused no matter which word you used.