The most important question to ask is "how much space do you need?"
For me, 64GB is too little. I have several seasons of several shows and films on it. Between the extended LOTR trilogy in 1080p, 3 seasons of Sherlock in 720p, 4 seasons of Waking the Dead in Divx, 2 concerts in DVD and a music folder of 20GB, I'm well out of space.
Now, if you don't bring that much videos, or you have no problems bringing an exFAT drive with you, 64GB is more than enough. 4.4 no longer supports apps to write to the SD, so using it for app files has become redundant. (Unless you root)
You're probably off cheaper buying two 64GB cards, keeping one in your wallet and swapping them around.
If like me you need that much space (And swapping cards on a train is hell), the 128GB might be worth the money.
As for quality, both are fine.