I think the free trigger is actually better than extort, since it doesn't force you to play off curve to get the benefit. Evasion is also good, Tithe Drinker has a hard time connecting.
These are fair points, but I'm thinking more of the format as a whole. White and black are often stacked with excellent 3s; I'm not sure how low-power you'd have to go for Drana's Emissary to be anything more than an on-the-wheel pick. Tithe Drinker is similarly mediocre, but lifelink on a chump blocker/extort engine is very nice in the 2-drop spot imo, and though it will often be an always-wheeling, end of pack pick for the player, it's far more likely to make the final cut of a draft deck simply because the drafter likely has more room for solid 2-drops (having typically picked a glut of flashy 3-drops). In other words: you'll take either on the wheel, but, assuming you're , I'd be surprised if the 3-drop can find room even half of the time, whereas I'd expect the Tithe Drinker to find room 75% of the time or more. Neither of these cards are strong enough to splash for, so I'm moreso considering what the deck might want, rather than which is objectively easier to get value out of in general (a calculation that will nearly always push the curve up if you're not keeping its containment in mind).