Depends what you're using it for. If it's just a groundsheet under a tent or bivvy, the lighter military surplus ones are fine they're tough, cheap, and you don't need waterhead ratings like you would for an actual tarp shelter. But if you want something to pitch as a rain shelter over a hammock or open camp, go heavier, 300gsm or ripstop, otherwise it'll sag and pool water in the middle after a night of rain. I've had a lighter tarp let me down exactly that way looked fine dry, then filled up like a bathtub by 3am. Eyelets matter too, cheap ones pull through the fabric under tension, so check how they're reinforced before you buy.
