playBunny: Well I can't remember where I originally saw that, some book years ago.

I just looked them up at bkgm.com. Apparently they use 46,656 iterations (seems like a funny number) and assumes cubeless play for a frame (which is essentially what we were talking about). There are probably more accurate rollouts available but they seem pretty good.

They say:
4-1 = 49.8%
2-1 = 49.9%
6-4 = 49.9%

Important to note that 6-4 has many gammon wins compared to gammon losses so is usually a better roll, but if gammons are not important as in the last frame then it's a bad roll.

Best roll in all cases is 3-1 obviously.

Also even 4-1 is winning if gammons are taken into account. Moving first gives you an advantage in most cases.