Personally, I wouldn't want to be without either my DA or my rotary polishers. I use my Dewalt or Makita rotary polishers for as much of the heavy work as I can, sometimes if it's the right paintwork I can finish out clean with the final polishing without inflicting rotary induced holograms. If not, I'll use the Griot's Garage 6" DA or Porter Cable for the final polishing step at a lower speed and use them to apply the wax/sealant. The rotary is a super smooth tool, if you center the pad you're working with no vibrations. If I'm working at speed 5-6 on those smaller DA polishers all day trying to compound and polish paint, my brittle hands even with some gloves get wrecked by the vibrations.
It's always a good idea if you're less experienced to start out with a Porter Cable style DA, with the right pads something like the Griot's Garage 6" is going to have the power to cut through heavy swirls and you'll get the hang of effectively working with polishes and compounds with a tool that is much easier and safer to handle. They aren't going to break the bank either, you can find them for great prices. And even at the enthusiast/professional level when you start to accumulate a larger assortment of tools, these smaller DA polishers don't become obsolete. They always have a place whether it's applying waxes/sealants, performing light polishing, using a brush attachment for carpets, converting to a 3" backing plate for spot polishing, etc. It's a purchase that lasts, and if you're not doing any sort of volume work and you're only working on your personal cars, you'll find that you're going to be reaching for them more often compared to the heavier tools. If you're properly washing the car you only have to compound the paint once. It can be maintained after that with lighter polishing tasks.
Now, if you are getting some higher volume of work, something like the rotary, the forced rotation Flex 3401 DA, or the long-throw 15-21mm DA polishers are going to be more efficient and effective at heavier compounding and polishing. They get the work done faster and are going to be smoother than the smaller DA polishers (unless you have the Rupes Duetto DA which is really smooth).
There are trade-offs with all of them though. The rotary is the most difficult to master, and it has the highest potential to damage the paint in inexperienced hands. It runs without vibrations, it's excellent with smaller backing plates for spot polishing, it cuts/corrects paint the fastest IMO and the Makita and Dewalt can usually be found at a much lower price compared to the Flex 3401 and long-throw polishers. The Flex 3401 DA is forced rotation, like the rotary this can make it more difficult to handle, and it comes with a high price tag. It's a great all-around tool for compounding, polishing, and even applying LSPs. It corrects fast, and it finishes well without having to worry about holograms, but it's not quite as smooth as the long-throws or rotary. The long-throw polishers like the Rupes Bigfoot or Griot's Garage Boss are easy to handle, easy on the hands as well with very low vibrations but they aren't forced rotation, you have to make those slight adjustments on curved panels to maintain optimum pad rotation, and they are also higher priced. You're also more limited compared to the others on backing plate sizes, if you need to do some spot buffing or want to get into tight areas you have to go with a different tool. But they can also cut quite well and leave a great finish without holograms.
So you have a lot of options to work with and there is no best option for everyone. What works best for my current budget and skill set is the rotary and smaller DA combination.
