I've always have felt that external drives are even cheaper than internals. At least, now in the few past 5 years. Storage options have incremented throughout the whole board. Now you have high capacity cheap USB drives, WD My Passport cheap drives, cheap external SSD's, high capacity microSD cards.

You can still go with the route of for example buying an OWC Internal SSD and use it with its case, but it would probably be cheaper buying a high capacity USB 3.0 pendrive, or even more practical in terms of packability and size (a high capacity SD or microSD card).

I, of course prefer the standard external SSD drive for proper storage. As it surely has to be a bit faster than an external USB 3.0 high capacity pendrive, but also because i dont lose it and feel its more solid to store things. That said, right now my daily driver are High capacity USB 3.0 drives as sometimes... well i need storage on on the go format.