Other thing is Qatar timings from CPT aren't ideal on the way back .. long day flight to Doha on the way back leaving CPT around midday ... and there's only one service a day ( same time every day ) which will only connect with the Dublin flight on certain days without a long layover in Doha ... Also yes your self connect at LHR will add to the timing given you'll need a longer connection but the overall flight time will be shorter as going via Doha is quite a detour ( you're looking at a ~7 hour flight and a ~10hr flight via Doha vs. a ~1hr + ~12hr via Heathrow )
Comparing the two services is a bit of a first world problems thing probably .. in my view I'd say BA on a good day clearly wins , but the key is that 'on a good day' bit ... Qatar more consistent generally in my experience ( though in the winter I think you get the older seats on the flight to/from CPT , so not as good as the Dublin leg on the 787 )