By appearance I tend to prefer the Vantage but I've seen some Vantages that had some real fit and finish issues. From a standpoint of what you get for your money, I think the Spitfire wins. You can find Spitfires in both full size and mini versions with S30V steel for less than $50. I think those flavor of Spitfires beat a Vantage every time.
If you compare the Avid Vantage vs. the S30V Spitfire it's really no contest (both can be had for ~$50):
Steel: Vantage-420HC, Spitfire-S30V
Lock: Vantage-liner, Spitfire-Back Lock
Handle: Vantage-Dymondwood/Plastic, Spitfire-Aluminum
Deployment: Vantage-Flipper (with "not great" action), Spitfire-Hole
I prefer the clip on the Vantage over the clip on the Spitfire, but that's really the only category where the Vantage wins in my book.