The "SOS" prosign is for use with telegraph (typically Morse code) transmissions.
"Mayday" is for voice (radiotelephony) transmissions.
Therefore, as long as your radio(telephony transmitter) is working, you should use "mayday" if in your judgement doing so is required. "SOS" should not be used on radiotelephony. Saying "ess ooh ess" or, Heavens forbid, Sierra Oscar Sierra, might get you some attention in spite of that, but I can see no reason whatsoever to go out of your way to use nonstandard phraseology when a perfectly good (and internationally recognized, and drilled into one's head from early training) standard alternative already exists.
If for some reason your radio isn't working, then squawking 7600 or possibly 7700 and simply executing the proper loss-of-communications procedures for the airspace you're in is likely to be better (and will certainly be more readily recognizable) than trying to signal SOS via an intermittent, unmodulated AM carrier, which will come across as little more than clicking to anyone listening.
Your radiotelephony training and examination should have covered emergency communications, including the use of mayday. Mine certainly did, both on the theoretical as well as the practical examination. Thankfully thus far I haven't had to use that in real life.
Compare the extensive discussion in Why do airplanes use MAYDAY when in danger but ships send SOS?