For me it boiled down to the cost of wheels/tires. Since they are a consumable item and the are considerably more expensive for a truggy, I opted for the buggy class.
Truggies do have a lot to offer - higher ground clearance, wider stance etc. And unless you end up upside down in the the turtle position they are pretty much self-marshalling!
If you are going to race, make sure that there are enough other people in your area with they type vehicle you choose. In my area for instance, buggy is king; often at club races they have a hard time making a truggy class.