How fast is 256GB to 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.