if you have complex models, than mdl is faster than wmp. but i think that mdl is generally faster than wmp and looks better, tough.

edit: i re-read your post and have a question: do you want to make a map that is completely out of models? if so, you may have noticed that you cannot walk on models. maybe you could make a "hull" of invisible blocks arround your models, so that you could walk on the models and have a better collusion system? i guess that it should work, but havent tried yet. by the way: the idea with the black lines in the texture sounds better for me, because its easier and looks better than a few hundred models sticked toghether to a level

ps: sorry for my bad english, i hope you could understand me