One card does not really outperform the other. The difference is not noticeable at all in 90% of games. Of course there will always be games where the 770 is ALOT better and some where the 280x is ALOT better. This is normal. Buy you cannot say one is better than the other. They are comparable.
Also, it is NOT $80 less. Come on. Not including mail in rebates, you can get a 280x for $289 and a 770 for $299. The price is MUCH closer than you are are making it seem. I like the 280x and the 770 as equals, but if there is only $10 difference, I would get the 770 due to less heat and much less power usage.