As much as I'd love to discuss this, I've been up for 24 hours.
Megabits have two different abbreviations, Mb/S, and Mbps.
There's the official abbreviations and the unofficial.
The unofficial is what's familiar to say; the official is what's right to say.
As I said, the most used term of MB/s is for megabits, at least in Europe.
Why? Because MB is an abbreviation, so we use uppercase letters.
S is the final letter, so It's common to use lowercase letters.
The result is MB/s.
It's differences between common/familiar and tech-common =)