There are three main types of resin-
Epoxy resin- this is the most expensive and the same stuff (different formulation) that makes up epoxy glues such as Araldite and car body filler. It is pretty inert. It can smell nasty but the casting resins usually manage to wipe out bad odour. You'll commonly find clear casting resins are epoxy but very few miniatures are cast in it. It is suitable for producing forms that can withstand high temperature such as for vacuum forming patterns and so does get used (often with aluminium powder as a filler) to make master casts of miniatures that are going into vulcanized rubber moulds.
Polyester resin- this is the cheapest resin. It is brittle, smells terrible, has a nasty habit of retaining or regaining tackiness if the atmospheric moisture level around it gets too high and dissolves polystyrene... avoid it at all costs!
Polyurethane resin- most commonly used to cast miniatures. This is often formulated with an easy 1:1 mix ratio, is pretty inert, doesn't smell (or not much) and is relatively affordable. Fast Cast is one example. Definitely your go-to for casting of that kind.
You'll also find much cheaper silicones, though again there's a bit of a range and you often have to buy in bulk. What you want for moulds like this are Tin Cure, also called Condensation Cure silicone rubbers. They will come in different shore hardnesses (kind of the softness of the rubber) and with different stretchiness but for a simple open mould like that pretty much any mouldmaking silicone will be totally fine. Again I favour EasyComposites, CS25, but have also used Silastic and Smartsil for bigger prop applications in the past.
Smooth-On and their distributors Bentley Chemicals are a brilliant source of specialist knowledge and products which I used a great deal in my propmaking career but their formulations are really overkill for such basic moulding and casting.
