Römer lists the R model under "Premium Line" and it also has the special side impact protection. I quote from the Römer homepage:
"In the event of a side impact, your child is optimally protected thanks to our side impact protection technology (SICT). On the one hand, the forces to which your child is then exposed are diverted away from him or her, and on the other hand, they are weakened. We have adapted our SICT for our child car seats. That's why it looks a little different on every child seat and infant carrier. But you can be sure: Your child is always in good hands in his or her car seat."

I dealt with the same question yesterday and bought the M model anyway.

Thank you!