The pair of HD-6970's are faster than a single 270 but they are not double the power, maybe about 1.6x faster. If they are willing to swap power supplies with you for a for a little cash it could be worth it.
You will need at least a quality 750watt or other 850 watt to run two HD-6970's while two HD-7850's are ok with a quality 550watt or most 650watt power supplies.
If you have enough power supply, the HD-6970's are faster. They will make more noise and heat.
It's not worth doing if you have to buy a power supply and extra case fans. Looking for a second used 270 would be a better choice.
-look up the wattage online or in tom's charts. Two 270's are about the same wattage as a single HD-6970. Look up some gaming benchmarks too, you will see the HD-6970 is not too bad, but it does use a huge amount of power to get the job done.