I'm going to vote that Kakistos was older.
I'm going strictly on names and appearances here. First of all, names: the name Kakistos is not only Greek, but also ancient Greek from an era in which surnames were not carted around around. The Master? That's a title, one very fitting for a feudal sort of guy from feudal times -- a guy who seems bent on creating a mini-empire on a small patch of Earth and, to help him run it, of siring those who will owe him reverence fealty. Ancient Greek versus medieval title? I'm gonna go with Kakistos.
Their different appearances do seem to be in-line with that time-frame. The Master still looks like a human (albeit an albino one), just like a human who is starting to lose his human traits to wrinkles and pointyness. Kakistos, with his cloven hooves and reddish skin, seems further along in that process.
Just my two cents. I think The Master is an itty bitty baby next to Kakistos.