If the class/grade ratings are the same in NZ as they are in the US, you should be fine in almost any whitewater-specific boat. The Mini Mystic is a river runner/creeker, and the SCUD is a playboat/creeker. For easier runs, the SCUD should be fine, but remember that it doesn't have the footblock system that the Mini Mystic has. You might also want to consider the Huka, if you don't plan on running any big drops.
The SCUD is more of a "If I can only have 1 whitewater boat", but it has a planing hull, which means that it will be better at playboating than a true creeker, but it also means that it won't absorb a landing like a creeker will, esp. when you land flat.
I would say get the Mini Mystic, since you can't really outgrow it, in terms of performance. The only problem with a boat like a SCUD is that you might eventually want to upgrade to a true creeker, as well as a true playboat, which means that you would then have 3 boats. It would be ideal for a raft support trip on a river like the Colorado River in the Grand Canyon in the US, since you can paddle down river, and still play.
Really, a playboat shouldn't be very much tougher to paddle down class II/III, except that it will be a bit harder to roll and the edges could get caught in the current, meaning that you will flip more. Some people will paddle playboats down class IV, as well as big water class V rapids. At my local whitewater course, the US National Whitewater Center, more people paddle playboats, just because there are so many playspots. It is class III on one channel, and class IV on the other, though.
Basically, getting a creeker will mean that you should advance faster, since it is more user-friendly than a playboat. Getting a playboat means that you could have more fun on easier rapids, but you would have a tougher time on some rapids. There are some people that use a SCUD as their main creekboat, though.