I cant comment on YSL, but I do love my BB 1818. If I am going into a meeting that day, or want to impress a big-wig, that is what I'll wear. Great longevity (I'll get around 5-6 hours), without having an overpowering sillage. It's goldilocks for me - just right. Love it.
Cant comment on 1818 but Rive Gauche has great staying power - plus the drydown smells great - I get a whiff of it in the evening even with 1 spray in the morning.
They're both great! They're not similar at all, though, and they're not priced the same, so I'm not sure why you're comparing these two alone. Have you narrowed your selection down to these two for some reason? Whatever that reason is might help determine which you would prefer wearing.
There's another thread right now where gents are describing their impressions of Rive Gauche. To me, the scent is a mix of floral, some clove and a little patchouli. I like it, but it's not an extremely distinctive scent.
To me, the 1818 is a strong mix of vanilla and baby powder, is very distinctive and I like it very much.
To answer your specific questions, I like them both, but have a strong preference for the 1818. I love the powdery vanilla, and it has fairly strong sillage and longevity (8 to 10 hours). It's also a scent you can wear anywhere without offending anyone around you.