Whatever main card you have I'd recommend having a backup from a different bank and of the other 'flavour' (Visa/MasterCard) in case one of those systems falls over, which seems to be not uncommon these days. And there's always the lost or locked out possibility of course. (I have a Tesco/MC as backup for my Visa in post #14. Both get used routinely in the UK, so are always ready-to-go.)
As it is only for emergency use abroad I wouldn't worry about it being commission free.