based on what my year's maitrise cohort ended up doing when they graduated last year, and based on a conversation with a partner, it seems appreciated by city firms. the partner i spoke to had done the maitrise and basically said that he appreciated applicants with the maitrise because he knew the effort that went into it. the maitrise cohort from my intake have all secured training contracts with big city firms or are in postgraduate study at prestigious universities (ivies, ox/cam, sorbonne). i can't think of anyone who completed the maitrise last summer who didn't do very well for themselves on graduation.
i've no idea about the impact for the bar. if you do well, it's obviously indicative of academic merit as you'll be dual-qualified. it'll also be a factor that can separate you from other candidates (which is, ultimately, the big hurdle atm as i understand - lots of exceptional candidates who have all mooted/got 1sts or high 2.1s/done minis/won prizes/pro bono etc). i imagine it may have more weight if you're looking at chambers who do international work that might require/find language skills helpful, because it'll demonstrate competency with the french language in a legal context.
my impression of the maitrise based on friends who have completed it and friends who have dropped out of it is that it's incredibly demanding compared to the llb - certainly at my university at least. i'd therefore imagine that anyone reviewing your application (whether chambers or a firm) who is aware of its rigorous nature would be impressed if you did well. but personally i would say it's worth considering that it IS a challenging course, and i know a significant number of people at my university who changed to the llb because of this.
looking at the two universities in terms of employability for the LLB only, i'd wager there's very little difference. what i am learning as a final year is that a good uni is a good uni - it's not going to open doors by itself and is merely one factor in the equation. it's more important to do well academically, to participate in ECs (eg. mooting, debating, pro bono, societies) and get work experience such as vac schemes and minis. if you're an excellent candidate, i doubt that going to exeter over durham or vice versa will be a dealbreaker (but ofc i am happy to be corrected if anyone can say otherwise - personally i see the difference as minimal in the end). neither will hold you back imo, but i am a lowly law student and therefore advise taking my views in that context
others may be more qualified to comment, e.g. former maitrisers or people with experience in recruitment. perhaps it's worth asking jess from freshfields for a recruitment perspective on the maitrise and how much weight it might be given?