It’s a toss up, my buddy has a quad and a mule 6 seater. The mule has more creature comforts and he puts a baby seat in it and the wife and kid and his family roll around, has a roof, windshield and soft side windows that zip open and roll up. The rear bench seat slides forward and locks becoming a 3 seater and making a bigger cargo bed in the rear.
If your hauling a lot of gear it’s hard to beat, you need a proper trailer and vehicle to tow it though. It won’t go down real tight trails like a quad will, I’d prefer a good quad over a SxS or Mule to be honest. I can quickly load a quad in the bed of my Toyota, it goes pretty much anywhere steep and tight, uses less fuel and carries plenty of gear on the front and back racks. My last two deer were packed out on the back of my 400 Kodiak with the rest of my gear on the front rack. It all depends on what you need it for, overall for me quads make more sense. One for me and one for the wife, we plow the property and pull a small trailer to do chores around the place with the quad with zero need for a SxS. I’ll be buying a 660 or 700 Grizzly for the next atv.
I almost think his mule is as wide as my 93 Toyota 4x4, it has more room in the cab than my truck but I honestly have been able to go anywhere I want to in my truck with the quad in the back to really get into the bush.