I’m surprised BA isn’t more price competitive on this route, given last time I flew to CPT it was ex-AMS via JNB on the A380 and they were much cheaper than anyone else - plus I wanted to burn a GUF2 before expiry.
In the OP’s circumstances I think going with QR is a no-brainer, particularly if the short layover options are available at the lower price (often they’re not and you’re forced into a 7 hour layover if you don’t want to pay lots more). I’m somewhat in the minority in that I’m not a great fan of the J lounge at Doha, although this may be clouded by either having a very short or very long layover at inconvenient times (just when you want to sleep you find there’s a horrendous wait for the showers), my transfer experiences have generally been a bit meh, and some gates are better than others for J boarding. But that shouldn’t detract from the on-board experience.
So if you’ve never experienced QR J then you definitely must - certainly if there’s an option with the A380 with the rear lounge, where you’ll find the interaction with the crew is much more akin to that which you’ll get with BA. I find QR crew much, much better than the robotic stereotype that seems to get unfairly attached to them, and as others have said the attentiveness is top notch - somewhat OTT at times, though stylishly so. The food is good, although my experience is generally restricted to the Arabic options so I can’t really comment on the more western options. Alcohol I’d put on a par with BA - champagne probably better, wine similar, spirits worse.