My preference for samples is at least a 1.5 mil spray vial. Without that much, and without the option to spray (I'm clumsy at dabbing) I don't feel that I've really tested the fragrance.
That said, the 1.5 mil samples from Perfumed Court are a good deal more expensive than the 1 mil samples from LuckyScent, and of course much more expensive than any that I can beg for free.
So I've been narrowing my choices with the free/cheap samples, getting a 1.5 mil to test again, getting a 5 or 8 mil to wear the scent for a while, and in theory someday before I die I'll buy a full bottle of something.
This is with stuff within a reasonable price range; the stuff that's offered in half- or quarter-milliliters to make the smallest sample affordable, I'm mostly ignoring right now. (Well, except for these three incredibly small micro-jars from Aftelier, sitting on my desk. And that drop of Black Cashmere pure parfum. And, well, just a few things.)
That's not to say that I don't have any full bottles, but all of them were either gifts, or purchased before my perfume obsession expanded and I realized that in self-defense I need a throttle to the buying process.