Graded copies in high condition are increasingly difficult to find, and the prices reflect that scarcity
There are admittedly distinct differences between Hordechief and Hellrider
I believe this is true regardless of how difficult they are to trigger, but emerging from Mortal Combat victorious seems like a breeze in a format like Commander where you can mill yourself to your heart's content with commanders like Grist, the Hunger Tide, Iname, Death Aspect (the mill comes from dumping your creatures into your 'yard on ETB), and Zask, Skittering Swarmlord
Choosing the right card size will depend on which style or kind of card you want to have, although cards that are made out of either thick cardboard or vinyl are the best ones out there on the market
Yawgmoths Bargain does cost a lot more than Necropotence, but the strength of putting cards into your hand for just one life means even at six mana its not at all balanced for its cost
