The Synapse 64GB has 32GB of 'usable' space, and 32GB of overprovisioned space.
The M4 SSD does not have any overprovisioned space, but you can always leave 32GB of space unpartitioned.
Overprovisioning increases the life and performance of an SSD.
The Synapse is for systems that do not have any other kind of caching available (Motherboard, RAID card, L2ARC, etc.).
The M4 is a standard SSD (like any other) which is only useful with some kind of caching available.