There are four base options for the MacBook Air - the first two feature an 11-inch screen while the others sport a 13-inch screen. There are only two differences between the two 11-inch MacBook Air models.
MacBook Air 11-inch 64GB vs. MacBook Air 11-inch 128GB
1) 64GB SSD vs. 128GB SSD
2) $999 MSRP vs. $1099 MSRP
The processor, RAM, graphics card, and all other components are identical for the two MacBook Air 11-inch variations.
As for the two 13-inch MacBook Air models, there are also only two differences!
MacBook Air 13-inch 128GB vs. MacBook Air 13-inch 256GB
1) 128GB SSD vs. 256 SSD
2) $1199 MSRP vs. $1399 MSRP
So, the question when deciding between the two 13-inch models is whether an extra 128GB is worth $200 to you. Usually Apple memory and hard disks are overpriced, but an additional $200 is pretty much in line with the market price for that amount of SSD disk space.
If you compare all four models together, you'll notice in the specs above that screen size, processor speed, battery life, SD card slot, and flash storage size are the only differentiators between the MacBook Air models. The difference in processor speed is small enough that it can be disregarded. When making your decision, first consider screen size and battery life (11-inch and five hours versus 13-inch and seven hours) and then decide on an acceptable SSD size.
The MacBook Air is the most portable Mac option available. The entry-level price point is also lower than any other Mac model besides the desktop Mac Mini. And you can upgrade the SSD in the MacBook Air if you outgrow it.
The MacBook Air might not be an option for some users because it lacks a SuperDrive (DVD/CD drive). However, an external Apple USB SuperDrive
retails for $79 for those users who really need it. MacBook Air users can also connect wirelessly to the SuperDrives of nearby Macs using a featured called DVD or CD Sharing.
The RAM is soldered to the motherboard, so it cannot be upgraded after you purchase the laptop.