"Silmar 41" is a specific pre-promoted clear polyester casting resin. "Alumilite" is the brand name of a variety of urethane and epoxy resins. Saying "Silmar 41 vs. Alumilite" is kind of like saying "Corvette vs. Ford".
In general, polyester resin blanks are harder than urethane resin blanks and therefore tend to be easier to polish. There is a wide variety of products, however, so that generalization is not always true. Urethane, for example, can be made into foam, rubber, paint, or hard plastic.
I have cast my own blanks using Silmar 41, Alumilite Clear, Alumilite White, and Alumilite RC3 Black - all with good success. Silmar 41 is polyester; the three Alumilite resins are urethane. When doing demos, I'd sometimes drop a Silmar 41 pen blank on a concrete floor and it would chip or shatter. I'd then throw an Alumilite Clear blank at the floor, and it would simply bounce around the room without breaking.
When I first started using Alumilite Clear, I ran into the same polishing problems as everyone else. The sanding/polishing process that yielded high gloss on acrylic or polyester simply didn't produce a nice shine on the urethane material. I resorted to the same solution as well - I put on a CA topcoat. As my buffing skills improved, however, I found that to be unnecessary. I can get a high gloss finish on all of the urethanes I use (including RC3 Black). I no longer put CA on urethane unless the blank contains an imbedded material that needs it.
I buff acrylic, polyester, and CA-finished blanks with Tripoli and White Diamond compounds. That yields a nice shine. With the urethanes, however, I also buff with blue compound and then with a plain Canton Flannel wheel. That gives me a glass-like shine even on RC3 Black. A light touch is needed as it is possible to buff ridges into the blank if you press too hard against the wheel.
I hope that helps,
Eric