How fast is 256GB to 128GB on mid-2013 MacBook Air? I've tried finding the comparison reviews of the performance differences between 128GB and 256GB PCIe SSDs of mid-2013 MacBook Air 11" and 13" (which now have been carried over as the early-to-mid-2014 MacBook Air 11" and 13", too), but some comparisons don't even mention which SSD they've tested, and others only present a single size for the mid-2013 Air models.

How much faster is the 256GB SSD compared to the 128GB on mid-2013 MacBook Air?
Sandisk/Marvell drives are about ~500MB/s write and Samsung drives are ~700MB/s write speed. Both have around 700MB/s read speeds.

It's important to note that while these are different, under regular use it's very unlikely that either will have any sort of performance implications. Benchmarks tell very little about day-to-day performance.