If you are human when you climb up the church, you will be able to see two summon signs, one right in front of the fog gate, the other one level down across from the ladder on the ledge. If you are human, you can summon both characters for the boss fight if you want to. You can only summon where a summon sign is on the ground, either from NPCs or other online players.
you do have to go back to firelink and talk to Lautrec before his summon sign will appear though.
I like Darkmoon blade because you get to PvP whenever you like, and its usually pretty active to find people.
the more important reason that this is a great covenant is that the items are fun to farm (in painted world), and the payoff (darkmoon blade miracle) is the best weapon buff in the game.