DSMS,
Wassup man! I think we were both in the same boat a few weeks ago wondering if the megs105 would work on flex, and i think you were also considering getting a rotary.
Well, i bought a rotary (hitachi) and used meg's 105. In my experience, the rotary is definitely faster, corrects more. The tradeoff is that you need to rack up some experience to get a really good finish (ie hologram free).
I tried the rotary + Menz 106ff on an Audi hard clear. Most of the swirls were removed, but because i was still not proficient with a rotary, i did get some holograms. The flex with Ultrafina + LC Gray pad fixed the holograms.
I think, with time and experience, i'll be able to tame the rotary, enough to get a good finish; and i think you will too!