The Plextor M5S series is in fact an upgrade of the existing Plextor M3 and M3 Pro models. Those SSDs have the older Marvell 88SS9174 controller, and not the new chip used for the M5 Pro zit. That 88SS9174 controller has been used in many SSDs, including the Plextor M3s.
The flash memory that Plextor used for thee M5S is 'standard' 25 nm ONFI NAND from the Micron/Intel factory. The M5S is available in 64 GB, 128 GB and 256 GB models. All three have an indicated maximum read speed of 520 MB/s, and the maximum read speeds are 90, 200 and 390 MB/s, respectively. The buffer memory of the three models is 128, 256 and 512 MB.
Since the M5S is positioned lower than the M5 Pro, the package it comes with isn't as comprehensive. All you get is the SSD, no brackets or migration software. The M5S also only has 3 years of warranty as opposed to the 5 years you get with the M5 Pro.
Hardware-wise the M5S isn't very exciting. We've known the Marvell chip for years, and we've seen the 25nm Micron NAND in dozens of SSDs. However, Plextor claims the M5S has improved firmware and is faster than the M3 SSDs. Let's see whether the benchmark results confirm that.