I'd Recommend Plaster

Well, I did it.  I conquered my fear of plaster!  It really was OK (mostly thanks to advice I got from some veteran teachers at conference this past weekend).  Here are some things I did that made it go smoothly (and some things that could have gone better):  I had cottage cheese containers half full of water at every table ready to go when they got in, I had small plastic baskets of plaster craft cut at each table (but not nearly enough, so I spent the entire class period frantically cutting more), I had four buckets/tubs of water at the back table for students to swirl their plaster-covered hands in before washing them, I covered the tables with paper for easy clean up (although I totally forgot to do this the second day) and I gave them three extra clean up minutes.  
Here are some photos after the mayhem, because I was too busy cutting more strips:

Smeary tables from the day I forgot paper.

Cups and buckets for water.

Plaster letters drying.

Room for drying is kind of hard to find

There is a dusty trail of plaster leading from my room through the gym and outside (to where there's also white grass from where students and I threw out the water buckets as to not stop up the sink).  The feedback from students has been very positive: they like how quickly it dries and how hard it is (compared to papier mache, which they have loads of experience with because I do love it so), and it's just something new and different and messy, so they like that too.  I haven't had complaints from the custodial staff yet (crossing my fingers and knocking on wood as I write that).
My verdict is I would definitely do plaster again (although I've never actually had to purchase it), because it wasn't as scary as I made it out to be in my head. YEAH for free plaster!!!!