Both are fantastic and at the same time very different. Guerlain's offering is fresh, while at the same time a bit darker, perhaps because of the tobacco and type of vetiver used. It also has an early '60's vibe going (at least that's what I get) and is perfect year round.
On the other hand, Creed's offering is much brighter and citrusy. It will make you smell and feel like you have a lot of money. Seriously. Their vetiver is different (Haitian), softer and cleaner, but I do get a very green, marijuana note in the drydown. In my opinion, this is a great one for spring and summer months.
If I had to choose between the two, I would go with Guerlain's offering. I enjoy it more all around, and it is perfect for just about any occasion, type of weather, or time of day. I owned both and ended up giving OV away; I've never missed it, as fantastic as it is. If you do really want both, you could always buy Guerlain Vetiver and pick up a bottle of Mugler Cologne, which is almost a dead ringer for OV (or the other way around, actually).