I have one of each. I would say the benefit of the onewheel is a much cushier ride (giant, wide pneumatic wheel vs 4 smaller polyurethane wheels) and more maneuverability. The onewheel can tackle all types of terrain, which gives you a real sense of freedom while riding, whereas the 4-wheeler is pretty much limited to pavement/concrete (although you can put 7" pneumatic tires on 4-wheel boards that can accommodate them). Onewheeling is much more akin to that snowboarding feeling except you can pivot more since you only have a single point of contact with the ground, but there's not as much (any, really) of the lateral slide that you get on a snowboard, which can be both a plus and a minus.
If I want to go fast, I definitely grab the 4-wheeler. If I want to take a more leisurely pace/go explore offroad, I take the onewheel. I totally get and agree with what @fosho said about having to be very nimble & quick in a pinch, which is definitely true unless you're not going that fast on it to begin with. I rarely, if ever, exceed 15mph on the onewheel, but I can easily hit twice that on the 4-wheel board. Having to dodge something on that thing at that speed can be just as catastrophic as having the onewheel turn into a catapult at 15 mph. Both have their risks, and almost all of them can be tied to speed.
huh, turns out orange man really was bad.