How do you tell this? Not that I have a 13" rMBP but I'm curious. What problems are you trying to discern by SSD maker
The OP is saying that the SanDisk SD0256F is slower than the Samsung SM0256F. Unfortunately, he's right.
My 13" MBA has a SD0256F and gets around 550 MB/s write, while my 21.5" iMac has a SM0256F and has around 650MB/s of write.
Meanwhile, read speeds are over 700MB/s in both SD0256F and SM0256F.
In real world usage, the difference isn't noticeable unless one writes large amounts of data daily.
We pay the same amount of money for a 256GB SSD, but it's like a lottery on whether we get the crappier SD0256F or the jackpot SM0256F.