If you want to sign up for a more set path and experience then go for Bain ("A Bainie never lets a another Bainie fail"). If you want to navigate and create your own path and experience then go for McKinsey ("Make your own McKinsey"). That is the advice I have consistently given to more than a dozen people who have been in your shoes.
Based purely on the fact that one firm is significantly larger than the other gives you that flexiblity as well as variability, both positive and negative, where you need to decide which options is more suited for you. If you want to maintain a global perspective than McKinsey definitely will have the edge with the scale and having one global P&L unlike the other MBBs. Your observations on economic sustainability is linked to McKinsey's global P&L as well as size and diversification of offerings. These are facts that I would not ignore as many of the other things are based on individual experiences, perception and/or reputation.
I'm surprised with your observation that everyone at McKinsey has only told you the positives. I know Bain have a very consistent recruiting message across individuals but McKinsey actually encourages the opposite where consultants are encouraged to share a balanced view including the things they don't like.
Despite being ex-McKinsey myself, my overarching suggestion would be to choose based on what you value most which is a decision only you can make. Here are my two cents on some of the themes you have raised:
+ Arrogance: this is commonly mentioned for McKinsey both in terms of the approach of consulting (i.e., maintaing an independent perspective - telling the client what McKinsey believes is that right answer vs. collabration) as well as (self-) perception of prestige
+ Hierarchy: I have actually heard the opposite, at least in London, where junior Bain consultants don't get much direct access to partners (one of my Bainie friends used to boast how he was working directly with a partner which is unheard of) whereas McKinsey it is rarely the case. I know of an ex Bain partner at McKinsey who has maintained the hierarchical appraoch and will rarely discuss the project below manager level
+ Prestige: generally I have found that people consider MBBs as one bucket of similar calibre but you will occassionally find executives and headhunters who will clearly differentiate McKinsey over others, similar to Goldman Sachs for investment banking, etc.
+ Powerpoint: you will be making lots and lots of powerpoint slides at any of the firms, unless you focus on implementation or digital consulting. The nuance about McKinsey is that there is a visual graphics team (mainly in India) who will produce (if you send them a sketch) and format slides for you which does save a huge amount of time!
+ Moments of stress: this one would be the same across both firms and would be purely dependent on the people you work. Both firms have upward feedback where there is an incentive for leaders to treat their teams with care and respect. The reality is not always the case but this is something I wouldn't worry at the stage of deciding on which offer to accept - you're not joining a corporate job where you have a fixed boss
+ Fun collabroative culture: at a firm level, Bain definitely seems to have a more consistent culture whereas you will find a Bain like culture within McKinsey as well as many others. Both firms will have the same fun as well as genuinely supportive people, fun parties and global events