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Typically I'd say MBB >>> MM IB (and even most BB's unless PE is all you care about), but in this case I think it really depends on how you feel about Dubai. If you for sure want to be in London, you should go to London. 

Not a hard choice to be honest. If you turn down MBB for Morgan Stanley, that's a close call. Turning down MBB for middle market banking is a loss for you. Sounds like you're at BCG, I know a dude from there who went to NYC from Dubai. They transferred 20 to NYC in total. If location is very important to you, be a good performer and ask for a transfer. MBB are generally easy with transfer, and the UK hands visas like candy.

Please no one answer OP until he gives rough salary ranges for each role. Don’t encourage people to not share financial info that is both interesting and helpful.