The 53918 is the unofficial "short barrel" load and uses the 5 petal Gold Dot which is designed open up faster. The 54234 is the standard load which uses the 6 petal Gold Dot. Both bullets are proprietary to the .357sig. A picture showing the difference is below. I prefer the 54234 version myself.
I wish TNoutdoors would do a test of the 54234 as he already tested the 53918.