The M2 version of the 850 Evo is exactly the same speed as the 2.5" version. They use the same controller. The advantage of the M2 is the small size for laptops and Ultrabooks. Now the fastest SSDs are M2 like the 950 Pro but it uses a completely different controller and protocol called NVMe. It just shares the M2 format but the 950 Pro is a completely different drive.
All things being equal the 1TB version should be slightly faster. Like you would need to run benchmarks to be able to see the ( tiny ) difference.