Congratulations on receiving two offers!
I can answer parts a) and d) of your question. In terms of future prospects and exit opportunities you need to keep in mind:
Where do you want to be 5/10 years from now - this could be in terms of geography, which brand do you see yourself working in, which values do you connect most to etc
In terms of brand: PwC is wider, Monitor is stronger (for strategy). PwC has its own management consulting arm, Strategy&. Strategy& can be comparable to Monitor when it comes to management consulting. PwC's inhouse consulting will have their own limitations in Middle East because Strategy& is very strong and can get better projects.
The kind of work you will do: It is possible that the top level strategy work would be done by Strategy& and they can pass on the baton to PwC's inhouse consulting for more operational/implementation kind of work. It is up to you if you find this kind of work more interesting. Monitor on the other hand will be doing strategy work as well.
Long term mobility: If you plan to move abroad in a few years time, you need to assess how strong Monitor or PwC is in that location too.
Finally, try to speak to as many as you can in both the offices and ask them about all these factors and things that you care about. This should put you in a good spot to make the right decision!
All the best!