Ive been using all of these materials for many years, here is my opinion.
Cold Cast, Resin, Fiberglass: (most expensive) This is a chemical plastic, there are LOTS of different options but the result is ths same process if you are constructing you will need to make a mold and then lay your materials to the mold. The pro behind this is strength and ease of repair. The cons are that you will be dealing with chemical reactions, fumes, smells and the process of making your own molds. There are some threads here in the forums that can cover how-to's regarding these materials, ALSO the smooth-on website has some GREAT tutorial videos that can be applied to a number of materials even if its not smooth-on. Bonus: you get to keep the molds.
--Cold cast: This method mixes some metal with the "gel" coating of the casting process. Typically powdered metal is added to the material and poured into the mold, some folks "dust" their mold for added coverage. This is where the con of cold casting is added to the over con of chemical plastics, if the powder coating is not thoughroughly mixed and the powdering of the mold not thin enough, the finished casting will have powder scarring or cure problems. The benefit is metal colored plastic that can be buffed and wont rub off much like real metal.
--Resin: resin is the binding material for all fot eh chemical plastics, resins come in lots of categories and formulas, resin casting can quickly produce results in a mold and is fairly straightfoward when working with it. The con here is that if done wrong, its a huge MESS, and waste of $$$. But if done right, its almost like set-and-forget it until its time to remove it from the mold.
--Fiberglass This is an added step to resining or cold casting where fiberglass material or cloth is saturated int he resin medium and added after the "gel" or "print" layer of the peice, it will add strength to your peice. Fiberglass is known for its resilience and strength when done properly. The drawback here is more money to make it stronger, but the peace of mind is well worth it.
--Resins over paper Some folks here will make there parts from paper models or templates on the site and reinforce with various resins and fillers and fiberglass. This can dramatically reduce the time you spend on making molds, the drawback is no molds to go back to if you make a mistake.
PVC (also Styrene?) (least expensive)Or sintra, or foamex, etc...
Foamed plastic sheets are cut to shape, heated and formed to whatever curve you want. Its less work to get the shapes and curves than scultping and molding. Its not nearly as strong or durable like fiberglass but works well. Downside is that due to its reactive nature with temperatures, really hot or really cold state will affect the material and it doesnt have "memory" so if it gets hot somehow and changes shape, it wont cool to its original shape. Sanding and getting smooth surfaces are easy as it typically comes in flat sheets. It is midly flexible so care needs to be taken when handling painted peices. It is lighter than fiberglass. Drawbacks also include limited thickness of materials.
Vaccu-Forming (middle of the road expensive): This process takes heated plastic and draws it over a shaped surface using a vacuum to have the plastic cool as that shape. Drawbacks are similar to sintra. Another drawback is that detail is usually lost in vacuuformed peices. It requires carving and a vacuforming table, sculpting.
Typically the quality of what you want in your armor is a personal preference, for the first time builder, most folks go with the sintra pvc armors and a resin or just plain fiberglass helm. Also keep in mind that on this forum there are numerous threads regarding all sorts of material construction.
It probably would be a good idea to think about where you will be constructing this and how much time and resources you wish to put into construction. If you are just buying these parts and not doing the construction part then a matter of quality material comes into play.
$$$Fiberglass: thickness and warping on purchased peices can be a problem
$$vacuuforming: thickness and quality of the plastic used will be an issue
$sintra: curvature and sizing may be an issue
Good luck!