HDPE 500 is the softer cheaper version, very shatterproof and the most easy (still not easy in itself) to weld.

It's what the Hannibalito's are made off.
And I think Hannibalito 3, build in 2007 and not spared in the amount of fights, is proof of the quality of the stuff as armor. On the other hand, it's in certain spots up to 30mm thick.

Polypropylene is useable, but it's harder and a tad less shatterproof than HDPE.

Acetal is even harder, but a lot more brittle.

RG 1000 , no comments there.