Hi, I'm in a similar position to yourself. I started windsurfing in August and am currently using a hired Rio S. I'm 83 kg and its easily buoyant. The M is a lot more stable when turning, particularlt in the chop on more windy days, but the S is my preference.
To give you something else to consider, early on I used a go 170 which I found better in every way, and much more fun. it was an older one with no daggerboard but if it were my money id bypass the rio and buy a go. Recent models do have a daggerboard if thats important.