As it stands, Tarnished Citadel is better. The reason being that your opponent can easily search out Mishra's Workshop, Strip Mine, Wasteland, Tolarian Academy, Library of Alexandria...etc
However, if you don't want to limit your opponent to just basics, but want them to tutor up duals, I suggest a templating along the lines of:
target opponent may search his or her library for a land card with a basic land type, remove it from the game, then shuffle his or her library.