Apple has sourced SSDs for their Macbook Air range from two different manufacturers. And one performs significantly faster than the other.
When iFixit tore down the Macbook Airs, they confirmed the use of Toshiba’s Blade X-gale SSDs. However, a thread over at Mac Rumors lead to the outing of a second SSD in later releases of the Macbook Air.
In the graphic below you will see two models, TS128C & SM128C. The Toshiba shows a read and write of 209.8 and 175.6 respectively while the SM128C (presumed to be a Samsung SSD), outperforms the latter with a massive 261.1 and 209.6 read / write.
Unfortunately there is no way to pick or choose your choice of Macbook Air when making a purchase. You will have to settle with whatever Apple has at hand when shipping your order.
If you own a Macbook Air you can check the revision number within System Profiler by going to > About this Mac > More info > Serial-ATA > Select the SSD > Revision.
If you have a revision number of AXM09A1Q you are in luck and have the quicker Samsung SSDs. A revision number of CJAA0201 means you have the Toshiba SSDs.