Do not underestimate the amount of stuff it is possible to get into a Prius, particularly if you put one or both back seats down. (It's a pity I won't be able to make it - I am a Prius packing EXPERT.)

If at all possible, I would focus on getting bodies into the van, and use the Prius for baggage - it tends to fit much more that way than if you're trying to get a mix of people and stuff.