I've owned both. Not to say that RFM isn't a good game, but it doesn't give me that "pinball" experience. Something about the holograms, the shortened playfield (I know it's not much shorter than a normal one but it feels like it since you can't see the back), and the fact that you can get very far in the game by just shooting up the middle.
I had my girlfriend play it for her first time (on default settings) and she got through all the modes and fought the mother-ship on her second ball. She ended her game with a ridiculous score like 10 minutes later (too many extra balls). The person I sold it to said the same thing - the day after I sold it to him he called me up and asked "I just fought the mother-ship, is it supposed to be this easy?" I know it's much harder to get to Attack Mars but I just felt unmotivated every attempt to get there.
Shadow I have also sold in the past, but I regretted it immensely after I discovered no game really compared. It has tough shots, really cool diverter ramps (never seen anywhere else in pinball) great mini-pf, awesome flow... it's the perfect balance of frustration, and fun. You have to earn what you get in Shadow and it makes you a better player.
Needless to say, Shadow came home again after finding one (difficult) and to this day I have no desire to play another game of RFM.