well if you coat something with resin and fiberglass, would it have the weight of resin and fiberglass? you answer that yourself
just resin gives you basic strength, but fiberglass gives you additional strength
both combined give you a good strength. if you need to compare it to something, its like when you are building with steel concrete.
Concrete alone has a high pressure tolerance, but tension, it doesnt take well. it goes the other way around with steel. combine these two, and you can build mile high skyscrapers.
the thing is, if you "coat" foam, the inside will still be foam. as long as the surface of the resin is thick enough, and strong, you wont have a problem. but if you press on it hard enough, it will still compress, as there is foam underneath.
also, sturdyness is relative.
if you treat something like foam good, you are likely to have it forever. on the other hand if you treat a resin/fiberglass piece bad, it will break too.
so it comes down to your usage, if you do a lot of action and stunts with suits, they will eventually break, the thing with foam is, if you paint it, and bend it, the paint will stay bent, so you will see imperfections. you cant bend the fiberglass pieces, or they will break.
you choose, whichever you want, and which you feel comfortable with, because noone can tell you what to choose