The Bucc18 class is the better organized -- Mutts are just now re-forming (from almost zero) as a class entity, very much under the wing of the strong & growing Bucc organization. There's room for both boats, and Mutts have always been embraced by their bigger sibling.
Buccaneer 18 The Sailing Experience
The Mutineer is, quite literally, the Bucc 18 with the last three feet cut off (and a shorter rig). No kidding -- they just took a Bucc mold & moved the transom in 36". Everything else is identical. That means the Bucc will plane easier, cuz it has those flat(ish) aft sections to ride on. It will plane upwind
with light crew.
It will plane in 8kts true, reaching. The Mutt is a far, far better boat for singlehanding or, as noted above, casual daysailing with people who aren't into: 1) hiking fully extended from the knees; and/or 2) swimming.
Here's the thing about both designs you need to know: the boat is almost a conic section & offers no
form stability. The Flying Dutchman (v. similar hull) puts the crew on a trapeze for righting moment. The Flying Scot has a ballasted centerboard. The Lightning has hard chines and
ballast. The Bucc has neither, and it's not a traps boat. So both crew have to hike hard upwind, all the time. Any winds over 15kts = survival conditions. The Mutineer a bit less so, but you are still talking about a boat with 8" of freeboard.
Other thoughts: both boats have large cockpits, but they are strangely crowded, so getting crew across on tacks or jibes in high winds is no picnic. The boom is much lower than a Snipe, for instance, and the CB trunk is tall. More than two people can be tough to manage unless reliably low winds. You cannot sit anyone low side over 5 kts. Believe me. The boats are very, very simple to rig and sail; tradeoff is very few sail-shaping or depowering options. You can roll up the jib (Buccs and Mutts have crude roller furlers), but the boat sails like crap under main only. Handles really good under jib only! For best control, make sure your main is crispy; use the halyard lock; and keep the outhaul as tight as can be. The vang depowers the main upwind; ease it slightly downwind to spill air from the roachy head.
So yeah, fun boats for very little cash. They require some level of agility, quick thinking, and a sense of humor. Dead easy to self right if you do roll one; I've stood ours up alone from full turtle in 25 kts, & I weigh about 140. Cheap to own, comfier than some boats in this category, and the Bucc & Mutt Class Associations are some of the nicest people in all sailing.
Upshot, I guess, is newbs will find the Mutt easier to learn on if the sails are in good shape. The Bucc has a much steeper and less-forgiving learning curve, but once you get it sorted, it's a hell of a lot more fun. Nothing to keep you from trading up once you get the Mutt dialed, either. These things are dirt cheap.