Diplomacy most of the time... you just can't beat the relations boost. It makes vassals easier to keep in line, and it makes any dealings with other nations easier. Your folks love you more, so you'll end it with most of their taxes and troops anyways. A larger demesne just means its more of my own money going into upgrading holdings.
Stewardship can be nice for the large demesne sizes, but honestly to get the best out of it you've got to be a wee bit gamey and revoke titles non-stop to insure you control most of your own lands. It's just not realistic, nor is it super fun. Still, there's times having a sizeable demesne is a life saver (wouldn't have survived otherwise as Nubia, absolutely had to control as many of my own counties/baronies as possible to squeeze out every troop I could until the Caliphate died).
However, I think people are over-looking one of the greatest things about a Martial education: you get a .5 health boost out of it. It means less unexpected deaths and more long reigns. Intrigue and Learning educations though... meh. Intrigue can be fun to mess with (especially if you're a vassal in a large realm, like the ERE), but Learning just kinda sucks for a ruler. Negative to fertility? Maybe if you're sticking to Gavelkind for kicks... and I suppose it's the way you want to go if your goal is to focus on tech.