I owned both. I sold the 3401. It's a great machine, just takes a lot more physical effort to correct with it. I've got back problems and didn't need it. The Mark II is easy to operate, very little downward pressure. I can speak for the ES, but I've had no stalling issues with the Mark II. I also have a G21, some stall issues with that when using the 6.5" pads. Not bad though. With a 5-5.5" pads no problems.
That was my experience.
I really loved using that 3401, because I find it to be a very refined, smooth operating machine. It was a lot of money to keep being set aside for a significantly lesser investment in the PC.
It's so much to do with technique, and personal approach though. I treat my orbitals like little rotaries, using the pad, abrasive, and if needed - high edge speeds to burnish away swirls. That thing can get humming if you're not laying down on it. The 3401 isn't able to achieve that edge speed. However, if you like, or are used to using a lot of pressure, the 3401 is fantastic!
The last time I used it, it was on a 2010 black Ford Focus, and this thing was absolutely trashed.
I started with a blue b&s polishing pad, and Ultimate Polish. Nope, not enough. Switched to green pad and Ultimate Compound - success.
I also tried orange and yellow pads, which also failed to match the green. Then I tried them with the 3401. When I got to the green pad I had high hopes, but I had to go over the same section twice, where it was once with the PC. I used the 7424xp for the rest of the car, polished it, sealed it, got paid, went home. That was when I decided to sell it. It went to a good home here in MI. If he ever sells it, it will be coming back.
Lately I've just been using my Rupes system, and totally loving it. I used it a lot over the summer, but now I'm really falling for it. So much that I'm selling some of my PC tools, some of my Megs excess product, and pads to reinvest in my Rupes arsenal.
It took me quite a bit to get really comfortable with the system. Mostly playing with the amount of product to put on the pad. Every day that I use the system, the more comfortable I am with it.
Regardless of what anyone here says, it's been proven time and again that both the Bigfoot tools, and Flex tools are 100% capable of performing to a professional level, and delivering world class results.
You could end up with both a Bigfoot, and a 3401!