I think 1 night is easier, but you have to take into account all the other DVCers that might be waiting for that 1 night as well to finish off their reservation. So it really depends where you are on the waitlist, like all WLs. But I think 1 night is easier to get than 7.
How many people cancel within the 30 day window though, I'm not sure. I tend to think your best chances were right at the 31 day mark. Our December 1 night waitlist came through in August (which doesn't support my 30 day theory, but anyway ...).