Nat 1st, you only get Nat as trade bait. Nat off lib, you get both as trade bait, and lib can be bulbed. It's a pretty viable choice if you have marble and the lib race is yours, but you don't have time for other things. Particularly if you have a gunpowder UU or want cuirassers ASAP or something, nat for the taj and drafting makes a lot of sense. Also, IIRC it is more expensive than lib, actually. Unless you clear the pre-reqs for astro, steel, democracy, or something like sci meth/bio, it's going to be the highest-beaker tech you can take.