I would think carefully about limiting yourself to 128GB internal storage - that feels tight very quickly in my experience, even without a lot of media.
Regardless, you will almost certainly want an external.
My first point - do get a portable one! I bought a desktop one with an external power supply for backup purposes, but having to take my MBA to the desk, where I don't work, find a spare socket for the drive etc. mean I don't back up as often as I should. Yes, they are minor inconveniences, but having any resistance to your backup and storage routines seems like a very bad idea.
Second - I also have a WD passport drive, and I like it very much. I also have a very good Samsung which I got from Amazon at an excellent price (in pink with hearts, but it comes in plenty of more subdued designs), and I would recommend either. TBH I would just go for whichever of the two I could get a better deal on, though I like the feel of the WD a little better (it's more 'compact' somehow). I've tried a Seagate, and they are nice, but mine at least is much slower in operation than the other two.
A good thumb-drive or SD-card is a good idea too for on-the-go storage and backup, and again can be had very cheap.