Silicone. Pure silicone, is odourless. take for example Tantus dildos (Completely smell free) and Lelo silicone (Again no smell at all.) but you pay more for it. Sometimes, pure silicone products can pick up a slight chemical odour from the packaging and glue used to make the packaging and for a week or so after you have opened the product, it might have a slight odour, but it vanishes.
However, those products are pure silicone. My personal opinion is that products with a smell, even though labelled "silicone" are actually either a cheaper silicone (not as pure) or they are silicone blends (Mixed with rubber or what-not) and yup, those toys do smell. The smell doesn't really go away.I also own a Doc Johnson Mood plug and I am not convinced that it is pure grade silicone, simply because of A) the price (Pure silicone is a bit more pricey) and B) The rubbery smell it has, which has not vanished, and I have had it months now. (Silicone doesn't smell rubbery) I have always suspected Doc Johnson use a cheaper silicone blend, as all their toys have a strongish smell and they are cheap to purchase. That is just my opinion though and obviously I have no proof.
I would recommend opting for a Tantus plug. If you want a pure silicone, odour free, non porous, silicone plug.
If you do go for glass or metal, I would recommend going a shade smaller than what you normally manage to take with silicone because, as you said, it is rigid and has no flex or movement or squish and so these toys can actually feel bigger and harder to insert than a product made from silicone of the same size and shape. For example, my partner and I can reasonably easily manage the tantus Ryder, with its 5 inch girth, but we cannot get the icicles no 25 inside, without like, half hour or more of trying. That is 5 inches in girth too. Go an inch smaller. Jenson is right though, glass feels much smoother and easier to insert in one aspect, and that is that it does not cause any dragging sensations, like silicone does. It glides a lot easier.