Sintra
Pros:
Easy to shape, and maintains its shape well through multiple troops
Takes paint easily
Thicker and looks a little more like armor.
light for size
Can bend to and past 90 degrees and retain sheep if properly heated while shaping
Easy to sand.
Thick enough to be beveled for nice clean looking edges.
Cons:
Harder to find than trash cans
Trash can plastic:
Pros:
Easier to find than Sintra.
Moderately cheaper than Sintra.
Cons:
Loses its shape over time (not all that much time-- you may find yourself reshaping it before each troop)
Doesn't hold onto paint as well
Prone to cracking when achieving tighter bends.
Most of the folks I've seen do the trash can plastic builds (tote bin builds) -- end up replacing their plates with Sintra/Foamex over the long haul.
Mando kits have to survive a beating across multiple troops, and are not single event kits. It's well worth it to make the effort, and intial financial outlay to acquire sintra/foamex, and avoid having to re-do later. (keep in mind paint has an associated cost too).
Cheers Vod!