I own R&M, and I've leaned both ways several times.
On one side, R&M has been finicky. I need to keep adjusting rails and switches to get it to play correctly. I don't have it working perfectly yet, but I'm working on it. It's so annoying, and the thought that I can sell it and get an extra $1000-$1500 now is enticing.
On the other side, R&M is pretty damn awesome when it works correctly. Some of the most fun I've ever had at times. It's challenging and I keep hitting that start button, so it definitely has that "one more time" feel to it. GnR has playfield issues. I'd hate to sell R&M to get GnR and put up with that. Plus, I'm worried that it'll be difficult to get my R&M back after I sell it. It's limited to 750 where GnR LE is 5,000 plus the SE's.
So I'm probably going to keep R&M and maybe review the situation again sometime in February. But it's close.