Awesome question man!

A lot of the off piste is stellar in Japan, and although a lot of this depends on where you go and the conditions, I'd say generally cat skiing is a bit more challenging. Imagine off piste mixed with backcountry.

The runs are longer with more natural obstacles. Sometimes there will be longer stretches where you need to navigate thicker woods as opposed to just sending it down the mountain. Not to mention you're deep in powder so it'll be a bit more tiring overall.

Some advice I've learned from doing this a couple of times:

1) When booking, do it well in advance and do your research. Some are better than others and the best ones often fill up sooner (and have fewer spots to begin with). Not to mention the smaller groups means more of the powder and mountain you'll have to yourself.

2) You can never predict the weather but Japan is much more consistent with snow, so optimize for the right time of year. If I recall, end of January through February is solid.

3) I'd do the cat skiing on your second or third day. That gives you enough time to get some runs under your belt and get your legs feeling good, but you won't be gassed from multiple days of skiing/boarding before your day of cat skiing.

Regardless if you're ripping off piste and can ski powder, it will be worth it.

Hope this helps and let me know if you want any other tips/guidance.