That is true, but not always. That is the contention of the bet. And yes, mostly [only ?] locals can take advantage of it.
Either way, and just for a bet. But you are right, I have to be careful. Let's just make it a gentleman's bet.
It's not my bet, but let's assume that you're inside NBBO on your market. It's more than likely that the natural will outperform the synthetic for a taker of liquidity. Of course the market-maker will earn more on the synthetic as he is adding liquidity at edge on two positions over one. So the bet is pointless and makes my point that this is a microstructure argument, not structural.
We can't trade the box and make the bet, so how do you propose marking the synthetic to spot? ES or SPY.
Hmmmm, I seem to have been shouldered aside in this mkt by experts... At any rate, I was just about to mention bid/offer (argument didn't take it into account originally), but nitro is willing to do it at mid. Now just have to figure out how to do it...