I'm stuck between the r9 280x and r9 280. The r9 280 is $50 cheaper and I am more on a budget. I could get the r9 280x and shift things in my build around. Which one is better bang for the buck.
The 280x isn't worth reducing your other components for the 3-5 FPS if you are on a tight budget. Use the savings towards a better motherboard (and power supply or CPU). You can easily OC a 280 , with MSI Afterburner OC software, to perform as well or better than the 280x at stock settings.
I would think less about bang for buck and more about what's the most I can get for what I'm willing to spend. Set your budget and find the card that best performs at that price point. For me it was ~$300 so I found an r9 290 used on amazon for $295 from amazon's warehouse. First one had issues; the second works like a charm. If you want price per FPS... Google and/or simple calculations can get you that, but it's not necessarily the way to go about getting your card.
The 280x is just a buffered 280, honestly, I'd go with with 280 for the price, you won't be much more out of the 280x imo. I'd rather you buy an r9 290 instead of a 280x for the price you'd putting for the 280x.