Was thinking of getting the base model with the 128GB ssd then get this
Toshiba SanDisk 512GB SSD for Mid Late 2013 2014 MacBook Air Pro Retina | eBay to upgrade the SSD.
The 128GB can keep until an external enclosure is available then use that as a external drive..
If you are like me and work on multiple Macs for work purposes, keep the 128GB internal with OSX on it.
Buy a 2.5" ssd and an enclosure of either usb3(5gb/s) or thunderbolt(10gb/s).
Install programs and store files on the external enclosure. Given that most apps are portable.
It makes OSX performance constant over a long period of time.
Allows continuity of work on different Macs simply by plugging in an external.
Sometimes I only carry around the ssd and a thunderbolt cable in a pocket.
If you are using for personal usage 128GB might be enough based on usage
It is more cost effective to get an external hdd.
However if you must, i wont recommend oem ssds.
They don't deliver as much performance for the same cost.
The only thing they offer is stability which is a grey area to be discussed.