Well since they are different types (one spring one PCP) I guess it depends on which one you want. If you don't have a PCP yet, you might go for the M-Rod. But then you'll need some form of charging gear (pump, tank hoses) to get started, which will cost more money. The good thing is once you buy the charging equip, you can uses it for other PCP's with little or no modifications except for maybe a different adapter/fitting. If you already own the charging gear, even better. The good thing about the R-9 is it's self contained and ready to go out of the box. The R-9 home tunes easily, with several aftermarket kits available if you ever want to try your hand at it. Any pro tuner can easily tune your R-9 if you don't want to DYI. So, really the question is what type of rifle will your next purchase be, PCP or spring? Once you decide that then the choice is (relatively) easy.
Nathan