Sounds like I'd really prefer a to get the M.2 Drive, but sadly I only have a mSATA port on my old mainboard. So I guess I'd have to either get the oldschool 2.5" drive or a M.2, with a PCI Adapter card, so I cantake it with me if I finally upgrade my CPU and Mainboard. And getting a mSATA drive seems pointless if I ever do that upgrade, cause moderne Mainboards don't seem to have that connector anymore.
So it's either M.2 with Adapter card for me or 2.5" SSD
I would go with the 2.5'' SSD in your case then. No need to worry about adapters and such.