Sintra is a name brand and not a product. it was one of the first manufacturers to make PVC so a lot of people call it that but it's not the product. I think you mean the difference between PVC and Styrene.
Styrene is a type of polystyrene, while PVC Sintra is made from expanded PVC. Styrene is generally more brittle and less impact resistant than PVC Sintra. PVC Sintra is more water resistant and has better dimensional stability than Styrene.
Different pallets of this stuff weekly. Mainly for indoor signage and posters.
.020" -.030" Styrene in dead white for two sided prints, .040", .060" thicknesses as well
1,2,3,4.5,6,10,12,19 & 25mm PVC, with the most common being 3mm & 6mm
If you have trouble with the corners cracking when they fall, you can round the corners and router again to help alleviate this. But you are correct PVC is very fragile and not crack easily when dropped.
Hope this helps.