It was a harder decision for me. I liked Paarthurnax, but seeing as how he used to be Alduin's second-in-command, I was worried that his apparent reformation was nothing but a very long-term plot to overthrow Alduin and take his place as king of dragons. Paarthurnax himself admits his own instinct to dominate. After what Delphine said, I didn't want to overthrow Alduin only to be betrayed by his seemingly charming former lieutenant afterwards.
So if you're playing Skyrim, here's a helpful poem
Only trust an elf or dragon, as far as you can throw 'em!