I would honestly vote towards sewing it. Not only would it be light weight, but you stand less a chance of breaking it. If it rips, you can just sew it back up. Getting the rifts and all worked out just requires a bit of tightening, or partitioning the fabric for separate stuffing.
I can tell ya now, Paper mache is the easier option, but it IS flimsy [my mask is made of paper mache, and requires inbetween con repairs] I dunno what this wonderflex stuff is, but if it can keep the mache sturdy, then paper macheing might be the better option
[center]Insert witty logic here.