I have flown on both EK
A330 and B772 economy, and theres not much to choose between them.
When i flew, B772 was a little more roomy, but the seats were great. A330 seats are also very good, though i found the ride on the b772 more comfortable.
However, the PTV's on A330 are better than the B772. They had a sharper screen on the A330. B772 PTV is a little bigger, but also a little less sharp.
Well, this was 2 years ago, so i'm not certain of todays standards, but EK
is always high quality for this sort of thing, so you can't lose either way.