Bristol. The only thing QMUL has over Bristol is it’s in London so a little easier for networking & work experience etc. But it’s really easy to get into London on the train.
Not sure which rankings you’ve been looking at, but while QMUL is reputable, it’s dropped off a bit the last year or so, Bristol is always a consistently strong contender in rankings which is key over a Uni dropping into the top 10 then bombing out again - and some do this.
But you also need to consider which you can see yourself being happy at. I think they’re both expensive to live at post year 1. But London maybe still slightly more, also QMUL isn’t in a great location. The campus is ok but the locality outside isn’t the best. I visited and didn’t like it tbh, but everyone is different.