I just flew BA premium economy and have flown EK in Y mostly and a few times J. BA premium economy has WAY more space than EK Y. For all the praise EK might get in Y it is still a typical long haul Y seat. I haven't checked, but wouldn't be surprised if the pitch on BA long haul economy was the same or very similar to EK Y.

Aside from space, I prefer EK service in Y. Drinks and food are more plentiful and more appetizing. I really like that EK serves wine in mini bottles allowing you to ask for a couple at a time meaning you won't run out before you're finished with your meal.

All told, however, the space takes the cake and I would certainly prefer BA premium economy over EK Y. But as skywardhunter says, check the prices for J. They can be very attractive and that experience is certainly well above BA premium economy!