Padang - a few different issues to tackle here:
1) Turning down a hypothetical Accenture offer to try for a role with MBB
First things first, do you stand a chance of getting into an MBB firm? You haven't provided any info that allows us to gauge this. Which university are you attending and what grade prediction do you have? If you aren't in the 2.1 from Oxbridge or First from Durham / Bristol / Exeter / LSE... category then choosing between the two options isn't actually reality and in that case you'd be a fool to turn down an offer from Accenture if consulting is the route you've chosen to go down.
2) Work / Life balance issues
I'll assume you are a credible MBB candidate for now to turn attention to the other issues that would otherwise be redundant.
Firstly all consulting firms struggle with offering work/life balance. The nature of consulting is that you are working on clients' most pressing business issues and so more often than not are working to deadlines that are pretty demanding. Sometimes the workload will be overwhelming for the duration of the project (I can certainly think of business turnaround projects like this); other times there will be peaks associated with milestones and / or crises on the project. But you'd usually expect to go weeks rather than months without having your evenings or weekends in some way compromised by your consulting career. Either because of working hours or because you're travelling and away from home (or indeed both).
MBB firms I would say are likely to expose you to greater work / life balance pressures, simply because there are fewer places to hide and a heightened expectation you'll deliver. Where a Big 4 type firm might have dozens of consultants out on a project, an MBB strategy firm will often only have half a dozen. Your visibility on a project is therefore higher - and so are your charge-out rates, by some margin. You'll also be surrounded by high-achievers, many of whom have a tendency towards sacrificing their short-term wellbeing for career gain, in part because the rewards are higher.
However it's not been my experience within consulting that different firms within the same class of consulting really differ that much in their work-life balance. At pretty much every firm you'd care to mention it'll be one of the top two or three reasons that consultants choose to leave the firm. It's also been my experience that there's a "luck of the draw" element to this as well. Depending on the culture that pervades your particular practice area in that particular office - and depending on what your managers had to endure when they were more junior - there can be differences within firms as much as between firms.
Your internship experiences to date worry me, simply because I would hate for anyone to enter consulting thinking that work/life balance isn't an issue and isn't something that you need to be very actively and consciously managing from day one in your consulting career.
3) Pay differences
Pay differences aren't as great as you might think - at least at the more junior ranks. Sure if you make it to Partner at MBB, you may well get there sooner and will be raking in more than if you'd done the same at one of the publicly traded major consulting brands like those you've mentioned. But very few people make it to these ranks. Below Project Manager level (ie. for the whole of your twenties), the gap to MBB remuneration is likely to be <£10k a year. The major gain from choosing the MBB route only comes when you i) progress to the more senior ranks or ii) get into a top business school on the back of your MBB experience or iii) get fast-tracked into a role in industry significantly above what you'd have secured had you not had MBB on your CV.
I hope this helps and good luck with mulling over your options.