what is the best thing to put on the floor of my vivarium?

Not sure what you mean by fake grass. A sort of reptile carpet? Honestly, I'd stick with the bark. First of all he's used to it. It will be better for holding the necessary humidity. It honestly doesn't make a difference. Neither provide significant benefits nor disadvantages. More a matter of opinion. It's not like either will effect the health of the reptile, but I personally think that reptile bark will look nicer, plus it does hold humidity well.

I agree with dill. The real problem with any of the fake grass/reptile carpet stuff, it's a pain to clean. Unless you get two pieces, use one while cleaning & drying the other.