In fantasy baseball, should the extra pitcher positions go to relief pitchers or starters?

Because you have Holds as a category you'll need to get a few more RP if you want to compete in that category. RP are nice because they can't kill your week with a bad start like a SP can, also they typically have lower ERA/WHIP an on top of that they play more games in a week. I would use 2 spots for pitchers that specifically wil get you holds and use the other spot for a SP. Good luck in your league!