Rio S or M ? Some advice, please.

I would like to buy a Rio for myself and my family. My question is : should I buy a M or an S?
I surfed quit a bit some 20 years ago (Tiga Fun Cup 360, only with daggerboard and old type sails), but my girlfriend is just starting. Our kids are to young now, but I would like to teach them in a few years too.
We took private lessons today on a Rio M (259x90 ; 2007?) with a 5m2 sail at 3Bft and it went really well for both of us. I think my girlfriend is not untalented since she managed to surf, tack, jibe and go upwind in 2 hours time and fell off only 3 times. She really had fun and wanted to go faster already.
I weigh 72 kg (158 lbs) and she 57 kg (125 lbs). Our weight has been stable for the last 20 years.
After my search on internet, I was tended to take the S. However, I asked my surf instructor and he said to take the M, since the S would be more for kids (?).
The shape and size of the Rio has changed since 2007. Can you give me some advice ? I would like to go as small as possible, but I don't want my girlfriend to be unstable on the board.