Simple to swap, a bit time consuming for backing up the initial one and formatting etc.
I'd go 2TB. My internal 1TB fills up quite fast. Seagate do a PS4 USB 3.0 external drive, if your not too fussed about price. It's supposed to work straight out of the box, format wise, and means you can nominate it as a secondary storage device. That's how I get captures/Gifs off of the console and onto my PC, and can use it as a back up too.
Your nearest Maplin might even stock them
That way my internal drive only has to store games.