I played both a couple weeks ago at Jackbar. R&M is the best offering by Spooky so far, but I still don't like it. I hate to say that because I love what they are doing. I really want to love one of their games. I think you would need to be a huge R&M fan for it to have any lastability.
GnR on the other hand is a total experience. Disclaimer, I like the R&M theme a lot. I've watched a bunch, but not all. I'm a huge GnR fan. Have been for over 30 years. R&M feels a little home made. GnR feels polished and smooth. To me it's not even close. If you take the collectability away from R&M I don't see much left.
All that being said, you can probably make all of your money back on R&M after experiencing it and sell it for a GnR once they become more widely available.