If you're going through the fibreglass route-
Epoxy resin has been another form of casting for stronger items with no flex (With a combination of fibreglass). Super rigid and light (Lighter than fibreglass resin casts) This is used commonly in gun stocks and some costuming companies where strength is needed. Macmillan, for example, uses epoxy resins in their rifle stocks. It's more expensive than fibreglass resin but is lighter and stronger I believe.
Fibreglass resin is an industrial form of casting. Basically, really strong, but if it gets crushed or dented, good luck repairing it. This stuff is mainly used on yachts, boats, caravans, cars (occasionally, before they moved to ABS) etc. Super strong with the best fibreglass to resin ratio. Absorbs most impacts but when it doesn't, you have a hole... Solution? Thicker wall. More resin, more fibreglass. Don't go overboard with the resin though, if you use more resin than fibreglass, you can make it too brittle. It's just like wetting a cloth, not soaking it. Soak it near the last layers to tidy up the fibreglass.
For helmets, I suggest using a two-part mold. Lay resin and fibreglass on both sides and let to slightly cure, before combining the mold and then strengthen the seams. Then, slowly build the strength of the entire helmet.
If you're going through the rotational casing route-
Smooth-On's 300 series has been the standard urethane for roto-casting for ages (Rotational casting, as in slushing the liquid around inside the mold)
However durability wise, you may want to try out smooth on's new 65D resin
It's basically a new and cheaper resin which offers better impact resistance than smooth on's 300 series. In other words, superior according to most sources. I haven't tried it myself but based off videos of stress testing. It's insane.
300 will typically shatter if you drop it down a set of staircases. 65D, with at least 7-10mm of wall thickness, should not.
The shore hardness is where things are a little bit more understandable. 300 has a shore hardness of 70 on the D scale. 65D has... 65 on the D scale so it's 5 less than 300 meaning it can flex a bit more, allowing better impact resistance, but it's not like vinyl so it shouldn't be flimsy.
Im not sure what the cons are comparing 300 to 65D. I believe that 65D needs to stay in the mold much longer as it takes a while longer to harden (Based off what I've seen). Removing it to fast can cause some warping issues as it would be soft and malleable.