For a helmet, it's a lot of work to get sintra to look good. Go ahead and try (I would use 3mm), but if I were you I'd save for a fibreglass bucket. As for the body armour, 3mm is okay, but it looks nowhere near as good as 6mm. I've gotten 6mm sintra to look as good as fibreglass armour. The thickness gives you a lot to work with. It depends a lot on how much effort you put into the process. While I'll still make 3mm armour for others, as long as I'm using sintra armour for myself, I won't use anything other than 6mm.