Hyperthreading for if the game supports it. That's about the only difference between the two companies. That and Intel tends to have larger caches which can also help... as to justifying the performance for how much more they are.. that's going to be on the consumer. Can you handle spending 400 dollars more on a 5-10 fps increase?
Issue with CPU's these days is some games work better with one chipset or another. And its not even between just Intel and AMD, but which Version. For example I'm running a Phenom II X4 970 Black Edition. Its an AMD chip. Not a new one either. However for me to upgrade, I would NOT use a next gen AMD, they actually perform worse in the games I play than my older CPU. Alot of it has to do with how they're made and much to do with their drivers being wonky.
You best bet is to find the game you wish to optimize your system for. Make a post in their forums. Ask about what people are using, seeing what settings they run at, their fps, and how often they crash. Then match their components.
For me:
AMD Phenom II X4 970 Black Ed. @ 3.5Ghz
ATI Radeon HD 5870
8GB DDR3 RAM
Windows 7 Home Premium
Runs FFXIV @ 60-75fps (45-55 in large fates and hunts) at max settings and no crashes.