It seems in the recent trend is actually "MS/s". Despite I don't like it (to me it reads "Mega-Siemens-per-second"!), I guess I have no better alternatives...
To my knowledge capital letters (like "S" for samples) are used only for metric prefix (M for mega, G for Giga and T,P,E) or for units derived from forename (Ampere, Kelvin, Pascal, Newton, Watt, Celsius, Volta, Ohm, Faraday, Henry, Weber, Siemens, Joule, Tesla, Hertz, Becquerel, Sievert, Gray). I would rather use Msps as an corect abbreviation.
If you look here (I don t know if it is not out of date) you will find there is no sample abbreviation, and see capital letter "S" is for Siemens and the tiny "s" is for second.
But as i said, i ve seen more MS/s than Msps, because if you are talking about sampling rate, I really doubt someone would traduce MS/s by Mega Siemens per second... It would be odd.