They are both excellent products. I use both on a regular basis.
I'll go against the grain and say that in my experience 476S is the better looking of the two. On both of my cars (which are both shades of gray), it just has a better look.
The first time I tried 476S, I applied by hand and put it on way too thick. It was quite a workout getting it off. I now apply via PC, very thin, and it is actually very easy to work with.
845 is a great product. It is so easy to apply and just seems to spread for ever. You'll be suprised by how little you actually use. Removal is just as easy.
Both product are very tough, and will last for months. IMO, you can't go wrong with either one.