polyester and polyurethane resins

It might be cheaper, more tolerant of mixing variations, easier to clean up, less hard on the mold, lighter (i.e the final product; polyurethane casts are fairly heavy), easier to work with afterward (e.g. cutting, sanding). If I had to guess I'd go with cheaper and less noxious/easier to clean. But, that's really just a guess.

I use polyurethane resin, which sets in less than a minute. It's actually pretty cool, you can see a cloud visibly expanding through the piece through the course of a couple seconds as it sets. I use Alumilite brand resin because that's what a local hobby shop had. There's cheaper stuff around, but I bought too much and will have it for a long time to come. I have also been using Alumilite's RTV rubber molding material, but I'm almost out of that. When I do run out, I'm going to walk down to the local arts store and see what other kinds of rubber mold supplies they have. I'm happy with Alumilite's resin, but the RTV is annoying. It doesn't keep very well (loses viscosity), and requires a 10:1 by weight ratio for best results. Something using equal parts by volume would be much easier to work with, so that and something a little cheaper are my primary search goals.

I figure it's probably pretty similar to what you're doing, but there's a short discussion of the process in this thread, and links to some photos:
http://bgdf.com/modules.php?name=Forums&file=viewtopic&t=1893&highlight=resin+casting

For a figure like that, it's ~$10US for the mold materials. I worked it out at one point for my little space marine dudes and it was like ~.07US' worth of resin per figure. For the Alumilite I use, $30 is enough molding material for 3 or 4 molds of that size. Another $30 is enough resin to cast in the high hundreds of pawns that size, maybe more. There's also $35 starting kits that have enough rubber to make 1 or 2 molds, and enough resin to make dozens or possibly low hundreds of parts that size, if you're careful and don't make a lot of mistakes. Remember though that the real cost is the labor. Pro casters tend to have a lot of equipment to automate things or a really streamlined process.

Rubber will carry more detail. You want to look for room-temperature vulcanizing rubber as your mold material. It carries detail so well, if you haven't smoothed the sculpt well it'll pick up on fingerprints & grease. Undercut (tight, acute angles, like under an arm) performance depends on the mold material. With weaker ones you can tear the mold trying to get parts out, but I haven't had a problem yet. Just realize that you have to be sensible and can't have really large overhangs and things like that.

Yes, and yes, but maybe not that many at a time unless they're small. I've made two parts in sizable quantities: the little space marines, and a set of small chits with designs on them (which I use for all sorts of little odds and ends, and as damage markers for LightSpeed, a cheap ass game). For the chits, I made like 20 initial "sculpts" (discs with patterns on them) and made a mold out of all of them. I just pour resin over the mold, sweep off the excess, and I get 20 little chits in a couple seconds.

For the space marine, I made an individual mold. Then I made 5 good copies that I cleaned up a bit. From those, I made one mold in which I can quickly pour resin into the five dudes. This is similar to what a pro caster will do to produce a mini. They make a mold by hand, produce 6 or 7 masters that they spend some time on to hand finish afterward, then from those produce a production mold which they actually use to conduct the run.

The pieces pop right out; if you were focused you could probably spend a minute per cast between mixing resin and popping pieces. With the 5 mold, I could make enough that one notable playtesting/design session featured us getting frustrated at a design problem and spending a good couple minutes hurling handfuls space marines at each other.

The problem with doing a lot of larger figures is that the resin will probably set before you can pour it all. Plus, the whole thing does take time; it adds up pretty quick. Also, after a while the mold starts to fall apart & the quality starts to suffer. When that happens depends a lot on material (for example, there's some RTVs that take much longer to set---48 hours versus 4---but are much stronger & last longer) & how well you treat it. I've had one become basically unusable, but it made a whole pile of those marines and I had made some mistakes in making it that made it a little weaker (as discussed on that link, I had let the two halves of the mold bond & had to rip them apart).

The resin I use produces hard pieces, harder than the soft plastic you find in the HeroClix or other pre-painted minis games, or those sets of StarWars games with painted figures. However, it is easy to cut & sand.

You can mix in other materials to control the finish & colors. I know Alumilite has stuff to produce metallic figures, etc, and I'm sure other companies do as well. I have a set of Alumilite colored dyes that I use to produce different colored figures. You just squeeze a drop or two into the resin and you get pretty bright colors. I've been very pleased with that. Once you figure out how much to use, you can consistently produce very good coloring.

I'm glad that's helpful. Let us know what you find out/wind up using. For pawns and stuff this kind of casting in small quantities really isn't a lot of work, and it'll blow people away so it's well worth it. A buddy of mine and I have run events at a couple local mini-conventions with prototypes of a game we've made using those space marines, and people were amazed that we had our own little figures. It was a big draw and we got a lot of solid playtesting from it :)