Man, what a loaded question! Lots of good points above, but in the end the formats are very different and very similar at the same time.
They both have the planning and preparation aspects, just applied differently.
Pacing is very important in both, but they're applied differently. In a hundie, you ask "how long do I think it will take me to finish" and apply pacing accordingly. In a 12 hour, you ask "what pace can I roll for 12 hours consistently" and apply pacing accordingly.
I think the mental aspect becomes very important once the race goes beyond 6 hours. Both formats go beyond that, but the mental games that get played are different.
The terrain in hundies tends to be more difficult than a 12-hour course.
I like hundies better, mostly because of the big loop format and the feeling of being really out in the wilderness during much of the race.
There's a couple of very well-run hundies that run through awesome terrain in the southeast, so there's no reason to try one out. You could also do something shorter such as ORAMM or the Fool's Gold 50. It's like trying out a 6-hour before jumping into a 12-hour.