According to Seat Guru, the EK A380 has a wider seat (18" vs 17.2") and slightly better seat pitch (32-23" vs 31") than the QR B787:
http://www.seatguru.com/airlines/Emi...irbus_A380.php
http://www.seatguru.com/airlines/Qat...eing_787-8.php
I have flown on the QR B787 on a day flight LHR-DOH and it felt uncomfortable after a few hours. However, the seat on the QR B777 is MUCH more comfortable (18.9" wide and 33" pitch):
http://www.seatguru.com/airlines/Qat..._777-300ER.php
If you find the EK A380 consistently comfortable, I am not sure it's worth the risk of uncomfortable flights on the QR B787.