I used the Rupes Bigfoot 21 with the 6" backing plate yesterday on my truck and I have to say, I was impressed.
Bouncers Capture the Rapture Monster Truck Detail
I'm a huge fan of the Flex 3401 though and it's two main best features are...
1. Pro Grade - If you're detailing full time for money, you should be purchasing tools that won't wear out or break if you drop them.
2. Gear driven, forced rotation and forced oscillation. You cannot stop the pad from spinning no matter what.
I prefer the Flex 3401 with the LC 4" backing plate and the LC 5" Hybrid pads. Set-up like this there's pretty much nothing you can't tackle and do the job right without any wasted time.
Tough choice, both tools are very different but also very good. Like Lost Highway stated, some people really like the Flex 3401 while some people don't like it. The thing is you don't know until you actually get one in your hands and buff out a few cars.
That's what's so cool about my detailing classes, you get to use LOTS of tools and work on cool cars and then you'll have a real world opinion that will help you to make up your mind.
Ah heck... get them both...