Urethane is urethane, however I would advise against pouring your own mounts too. I've seen some pretty ghetto shit in my day. A dude on zilvia used the rubber from a rubber mallet to make his motor mounts complete with the wood he broke off from the hammer still sticking out the side. I'd inject stock mounts with window weld before bolting hockey pucks to my car...

I ran JGS tools semi solid urethane mounts on my car for two years. They were SUPER stiff, I actually didn't have too bad of an experience with them because my old car wasn't the typical rattly piece of shit 240sx that's become all too familiar these days.