For 2012, QSDP is meh. Not quite as bad as 2013 where it falls off the cliff of good savings, but it's still a pretty narrow margin.
With that said, especially in a mixed group such as yours, if you're taking care of the check, then QSDP will be the way to go. You'd probably get away a bit cheaper just paying OOP for the meals, but the peace of mind of having a static budget for x amount of stuff in your case is likely worth something.
I'd do a couple TS to get everyone together (maybe start, middle, end of trip) and then let everyone do as they wish.
Now, again, to highlight the narrow margin we're talking about here, for 2012, the "break even" point on a single QS meal (entree, drink, dessert) is about $15 (a little less). The average cost of a QS meal (incl. entree, drink, dessert) is right around $15, a bit higher or lower depending on where you go. As such, there will only be minimal (if any) savings from going on the plan. (Note, that break even point for 2013 jumps to over $17, which is what makes it such a bad deal!)
So, from a purely financial standpoint, unless everyone is going to eat all the CS desserts, I can't recommend it for this year (or next year at all). But you have to decide if a set budget is worth the potential of paying a bit more. (Again, in mixed groups such as these, that does have quite a bit of value, which is why I bring it up a couple times

)