So 128 is pretty much bare minimum for everyday usage? What do you use it for and what kind of stuff you store in it?
First off, I think there is generally a loss in GB when you actually use the drive. They usually advertise the drive in bytes, so 128,000,000,000 bytes. 128,000,000,000 bytes / 1024^3 = 119.2 gigabytes. So you lose 9 GB off the bat.
I would say it really depends on your practices. If you strictly keep all media (pictures, movies, music) on an external drive, then I would say that you're okay.
You also mention games, which varies greatly. I think I remember World of Warcraft taking up 45 gigs before I uninstalled it. League of Legends is around 4.5gigs. If you've got a couple of MMO's running on there, you're probably using up half the drive. I would still say that you should be okay, if that is all of it.
The only other factor is how much other stuff you install. I'm guessing everyday use doesn't mean Photoshop, CAD, or anything like that. So once again I'd say you should be okay. And if you didn't go heavy on the games, you have some leeway here.
That being said, if its a difference of $40 between a 128GB drive and a 256GB drive, spend the 40 bucks and future proof yourself. If its $75 for the smaller drive and that's all you need, then go with it.