Phil, thanks very much! I won't claim to know everything there is to know about resin casting and I'm sure that there are many on here who have been doing it a lot longer than me. However, with wanting to do several different things with resin (cast pen blanks, cast jewellery blanks, cast various things in pen and jewellery blanks, including wood), I've done lots of research and a fair bit of experimentation.
Unfortunately, I've found that there is no perfect resin, each one has it's own drawbacks and limitations.
Polyester is by far the cheapest. It's also one of the easiest resins to use as it doesn't require pressure or vacuum, nor a narrow temperature range to set in. The downside with it is it's really strong smell (I don't find the smell too unpleasant, it's just overwhelming if you don't have good ventilation) and the fact that it is very brittle and hence not very strong. The brittleness is a deal breaker for me, as the dip pens I make have no internal structure, so the strength of the resin is imperative.
Epoxy resin is another easy to use resin. It's a lot stronger than polyester, is usually water clear and generally doesn't need pressure or vacuum - though this isn't always the case (I'm currently using a fairly fast setting one (we're talking about a 30 minute work time, but still around a day to fully set) and that will be full of tiny bubbles if I don't degass). It is also usually fairly forgiving of temperature. The downsides are that it needs really good mixing (not a big issue, but it makes my hand ache), is a LOT more expensive than polyester resin and can be difficult to polish up if you use a method which generates a fair amount of heat - and regardless, it will never polish up as nicely as a commercially made acrylic blank. It will also have a tendency to distort of you generate a lot of heat whilst turning. It also generally takes at least a day to set, and it may be several days until you can actually work it with power tools. This slow setting time can also be an issue if you wanted to suspend anything within the resin - mica powder is generally OK, but something like glitter would sink to the bottom.
Polyurethane is generally the preferred resin for making woodturning blanks, but again, it has drawbacks. There are also differences, depending on whether you want waterclear resin, or opaque resin.
I'll address waterclear polyurethane first...if you want a waterclear resin, it is expensive - around the same cost as epoxy. Secondly, the fumes given off by the waterclear polyurethane resins are toxic, so even though you can't smell them, you really need to work in a very well ventilated area, or wear breathing apparatus. Thirdly, it is very picky about the conditions in which it will set. Each different resin varies, but generally it has a very narrow temperature window (usually around 22-25 degrees) and will only set in certain types of mould - polypropylene (the type of stuff many chopping boards are made out of), or addition cured silicone are generally recommended. Fourthly, despite what the manufacturers may tell you, if you want bubble free castings, you will almost always need to use either vacuum or pressure - note that if you're making your castings opaque with something like mica powder this might not apply, as the bubbles are tiny....but if you want a totally clear casting, you'll need to get rid of them. Finally, and probably my main issue with polyurethane waterclear, is that I have yet to find one which polishes up well. Yes, you can polish it, but you will never get the glass like finish you can get from commercially made acrylic blanks. The benefits of polyurethane are that is generally sets pretty quickly (there are lots of different resins out there with different setting times, so it's fairly easy to find one which suits you), is strong, and turns pretty well (like epoxy, it will distort with heat, but is a little more forgiving).
Opaque polyurethane is generally about half the price of waterclear, and is usually more forgiving and easier to cast. It generally has a much shorter working time (2 minutes is not uncommon), and you may be able to get away without vacuum/pressure (there will be tiny bubbles in there, but they're generally not as noticeable in opaque resin). Because of it's shorter working time, it can be difficult to make multicoloured blanks, as you can't get them all mixed in time, however, there are a couple with longer working times if this is important to you. You also can't use pearly mica powders in the opaque resins. Well, you can, but they only give a light colour, you won't get the shimmery, pearly effects. The opaque polyurethanes are generally a lot softer than the waterclear polyurethanes, and you will find they distort at high temperatures. However, they are lovely to turn, and do polish up very nicely - probably the nicest of all the casting resins.
Bottom line is, you need to decide what factors are important to you, and choose which resin to use accordingly. Some of the issues I've highlighted might not be an issue to you - such as the resin distorting at high temperatures. As I've said before, I make dip pens with no internal support structure, so if a resin distorts whilst I'm turning it, it's a big issue. When turned on a pen tube it probably isn't an issue at all.
And as always, these are my experiences, your mileage may vary
