Please do a search on these forums for flex vs rupes. There are many many threads addressing this exact question. Generally, you get two opinions - Flex won't stop spinning no matter what is thrown at it and other than learning some technique to not have it walk on you it corrects/polishes finishes incredible. Rupes, GG, if you have the technique, works just as well and maybe faster/smoother on the hands. Either tool will do the job but Rupes / GG is a system as pointed out by Mike Phillips recently.
My path was with the Rupes but I just couldn't figure out the technique for vertical/curvy panels and then I purchased the Flex, which has been incredible. Maybe get the Flex book and video before you do anything and then research the Rupes/GG forums to get different ideas. I haven't seen a GG or Rupes book or instructional video yet and having to learn the technique is something you should consider.