I can vouch for the 256GB being faster than the 128GB for the 2015 rMBP 13". I bought a 128GB model first, then decided to go with the 256GB. But before I returned it, I ran the BlackMagic Disk Speed Test.
The reads speeds were pretty much identical, at around 1,400 MB/s, but the write speeds were double on the 256 model (~650 vs ~1,300 MB/s). I'm guessing this is due to the 256 having two 128 modules that it's writing to at the same time.
Whether there is any real world noticeable difference between 650 vs 1,300 MB/s is another story. Moving large files around, probably. But day to day, I'm not sure.
The reads speeds were pretty much identical, at around 1,400 MB/s, but the write speeds were double on the 256 model (~650 vs ~1,300 MB/s). I'm guessing this is due to the 256 having two 128 modules that it's writing to at the same time.
Whether there is any real world noticeable difference between 650 vs 1,300 MB/s is another story. Moving large files around, probably. But day to day, I'm not sure.