Are you considering only how each will affect poison damage?
If so, then from what I understand the answer is "it depends."
What it depends on is you target's Poison resist.
If is high, The -15% resist +15% damage will be better.
If it's low the +50% damage will be better.
Bramble damage = Basex1.5x(1-Resist/100)
Nat's damage = Basex1.15x[1-(Resist-15)/100]
Setting Bramble damage = Nat's damage and A little algebra and we find that...
The resist breakpoint at which Nat's Overcomes Bramble is... 6-7% resist.
So if your target has poison resist more than 7% then Nat's is your choice.
If they have less than that (Hell penalties, Facets in other gear, 3-piece Trang set), then Bramble will add more poison damage.