I've tried both. The CdG has more of a citrus bite at the opening, and feels much woodier than the Zara, which I find more floral and light in comparison.
I just resprayed them and there is a big enough difference to own both IMO. I was tempted by Vettiveru, but chose not to buy as I had the Zara and Guerlain. But I think I do still want it!
Edit: would like to add that the Vetiver note in the CdG is nuttier than the others.