I personally prefer a transit time of around 3 hours (to allow for delay, time for shower and meal without feeling too rushed) so I would choose the longer delay but ultimately it's up to you since I know there are many who would rather have a short or even no transfer if that's possible. I have had transfer time of just 50 minutes between flights at DOH in the past so baggage shouldn't be an issue with 90 minutes transfer if the inbound flight arrives on time.

Like you said, ex-LHR flights are prone to delays for various reasons (and probably more so if it's an A380) but my experience with QR is that the actual flight time is usually much shorter than scheduled. You also need to consider the possibility of bus gate which can add another 10-20 minutes to boarding / leaving the plane.