[thejeff]

Of course, while you may make up the cost after 9 turns, you've also got 18 less rp out of him. And he's more likely diseased and dying (unless you've got boots of youth on him)

I'd build whichever looked more useful for other purposes. Mostly warlocks since they get randoms so you'll want a decent crop to get the useful picks.

Cost efficiency of research is really more of a factor with non-capital researchers later in the game. Early on amount or research and usefulness of the mages trumps it. And of course what you can afford to invest in long-term goals and still expand/fight.