I think a waterproof sheet isn't as waterproof as a rain sheet.. if that makes sense. I also think that a rainsheet would last longer and stay more rain- proof a lot longer.
Here's what I got on dover for searching 'rain sheet'
A rain sheet would be sort of like a rain coat for a person...I've heard from alot of people that they don't like them 'as much' as a good waterproof sheet. The ripstop material in alot of waterproof sheets are a winner. Rain sheets are really expensive as well. I know for myself, I prefer the waterproof the rain sheets; they seem to hold up way better with regular turnout. Again, that's just my experience (and friends), so depending on the brand, and what not, your results might be different.