cptzippy,
To paraphrase the famous line in Jaws*, "You're going to need a bigger polisher!" Or maybe not... I don't have any experience with the polishes that you're using. For the MDX, you may be fine. But for the Jag, you'll need something more serious, IMO.
As a starting point, I'd suggest trying a different brand of polishes with your current polisher. I prefer Menzerna polishes. Mine are the old style - IP (Intensive Polish) and FPII (Final Polish II). Currently I think these equate to IP 2000 - Intensive Polish PO91E and FF 3000 - Final Finish PO85U. Here's a chart comparing the agressiveness of several polishes including Prima and Menzerna:
http://www.autopiaforums.com/Todd-Helme/polish-chart.jpg. This is a descriptive page showing the different menzerna polishes:
Menzerna USA: The World's Finest Polishing Materials. Here's another comparison chart:
http://www.menzernausa.com/Charts/Liquid Summary 2012.tiff. I think you'll want something pretty aggressive like Menzerna SI 1500 - Super Intensive Polish PO83 or maybe even PG 1000 - PowerGloss S34A. You might try a new polish before getting a new polisher.
Next is the question of the Flex 3401 or the Griot 6" polishers. From what I understand, both have an 8mm orbit. (I know the 3401 has that orbit and believe the Griot's does also.) That's one of the things that make them aggressive. OTOH, the Flex is a forced rotation DA whereas the Griot is a standard DA. The Flex will be more aggressive with foam polishing pads. (However neither are rotary polishers.)
I just ordered a Flex 3401 Sunday after reviewing a lot of polishers, to replace my current PC. So I'm a bit biased. But the 3401 has the power, features and handling that I want and the Griots did not.
One less than obvious feature on the Flex is the variable speed trigger. This means that you can feather the speed for the more sensitive spots and then go full bore on the tougher spots. I never appreciated that feature until I got a Mirka Ceros sander. Instead of fiddling with a speed control wheel, you just set the maximum speed and then feather the paddle when needed. It makes for much more efficient sanding and I understand that the same applies to polishing. I believe the Griot's polisher does NOT have this feature.
Here's a comparison of the Flex vs the Griot's polisher:
Flex3401 vs Griot's Garage 6" ROP - Auto Geek Online Auto Detailing Forum.
Good luck with your decision.
Regards,
Dan.
*
You're Gonna Need a Bigger Boat - Jaws (4/10) Movie CLIP (1975) HD - YouTube