Comparing Peacekeeper and Preacher isn't very fair, as both are heading for opposite strategies.
Preacher forces your opponent to overextend the board in order to pass through him (or they'll get chump blockt by their own fellows). As you mentiont early trading x-1 with a card like Wrath or Explosiv is the nuts and Preacher basicly drives your opponent directly to this point.

Peacekeeper on the other hand is great if you want to freeze the game in general, however creating a board state where only 2-3 cards in your deck can win doesn't seem to great. What I do like about Peacekeeper is that you can decide if he'll stay on board or leave, being able to set up a small army via Decree or Elspeth. Yet I don't like the fact that he's incredible slow, which isn't exactly promoting for a deck that is rather slow (assuming that your opponent can just outstall you, as you need several turns to find Jace, find couter to protect him, set him at 13, use his ulti and then wait 7-8 more turns for them to die. I know this is a normal way to win with Jace, but if your opponent can't win anymore there are only 2 possible things to do: surrender or stalling).

Speaking about the Merfolk MU, I'm looking forward to test Llawan in the spot of my 2 Preachers once more, as more and more lists including 2-3 Kiras have shown up in my meta recently.