I do love Pixie and all of the Tech4Learning tools. And, I have Pixie at home and my kids love it. So between those two, I would say Pixie.
However, a free, web-based alternative is Kerpoof.com. Maybe you have tried it. My kids (my own and my students) love it! It's free but you do have to make account in order to save creations. I use the "Gmail trick" to make a set of classroom logins. Give it a shot...nothing to install and it really encourages creativity.