From my experience:
I have always worked with the Makita rotary, with some Porter Cable orbital action thrown in the mix for finishing on some very soft paints. These two I used for about 3+ years on all the detail work.
Last year I picked up the Flex XC340 with 2 intentions: #1 To use it on some very soft black paints (Nissan, BMW, Porsche, especially Porsche) that I would get on a regular basis. The point here was to use the Flex when the client wanted a 1-step correction, because the Flex would do more correction than the PC but wouldn't leave holograms like a rotary does. I have finished down on these paints with a rotary before, but the fine polishes and pads required to actually finish down hologram free didn't do much correction, so Flex was the smart choice here. #2 reason was to train a helper/friend I had so that he could sometimes help out with polishing as well, as I don't trust anyone to use a rotary on my clients' cars' paint.
That said, Flex has replaced the rotary for me in many ways. It corrects VERY well with 5.5" orange pads and Meguiar's #105 and also finishes down great with M205 and a white or black 5.5" LC pad. I started using the Flex as a finishing step after using a rotary on jobs where I did 2 or more polishing steps. These days I would say it's 50-60/40-50 using Makita/Flex vs Flex/Flex on 2-polishing step detailing work.
Needless to say, Flex has completely replaced my PC for finishing. Actually, it has pretty much completely replaced the Makita as well in the finishing department. However the PC is still a great tool in my arsenal for work where 3" pads are necessary (HUGE negative for the Flex not to be able to use smaller pads) as well as applying sealants.
Long story short, for side work, especially if you're looking to do quality work, I would definitely recommend buying the Flex as the only tool. If it's possible, I would obviously recommend all three, but I can easily say I use the Flex the majority of the time polishing.