I do this trip a few times a year, and I always use QR where I can. It may add a couple of hours to the journey length but its ... cheaper, more flexible, more TPs, has QR lounge at LHR (better meal pre-flight|), Al Safwa lounge at DOH outbound, good duty free shopping at DOH on the return...etc

I might get fewer Avios, but I'm not short of them.