Heriot-Watt lacks international accreditation so QM is a better choice. Also see the FT online MBA ranking for even better programs

I am not in a position to comment on the comparison of these two particular schools, however I can say that I would not rule out any UK university/business school solely on the grounds of professional accreditation (or say international accreditation if you like). I would rather take that into account, but along with more in-depth investigation. The reason is simple: 1. Pursuing this type of accreditation is completely voluntary for business schools, depending on their strategy, 2. Almost all officially recognised UK unis (actually no-recognition would make any UK uni degree invalid) benefit from a sound international acceptance. The QAA also has international connections.
We can also spot some conflicts between some rankings, which can be considered more about the ends, with all their pros and cons, and the status of accreditation, which can be considered more about the means. For example, not all business schools that make it to the QS Global 250 list have this type of voluntary accreditation.

That is exactly why the QS list is not widely cited by business schools or used very much by students. Any ranking that has the EU Business School above Bath, Durham, HHLetc and which lists "Helsinki School of Economics" - brand which has not existed since 2010 - cannot be taken seriously.

QMUL will definitely be a better known brand, both in the UK and the rest of Europe.

However, its MBA hasn't launched yet. It's hard to judge the value of a course when we don't have graduated classes to look at.

Are you considering any other schools (Bradford, Warwick, etc.)?