For the above average user 50gb seems to be more than enough to last for a month's activity especially when you consider that you probably won't need to use your mobile data for that entire time if you have wi-fi. The Samsung Galaxy S4 has a microSD card slot that doubles as a mini SIM card holder, so you can use it as a dual SIM device if you want.
This is great if you have to make phone calls while out and about, as it means you won't have to carry around a second SIM card as a back-up, plus you'll get to use one of the microSD card slots for that.
However, you can only use the microSD card slot as a SIM card if you have a microSIM card, which is smaller than a regular SIM card.
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