Synthetically equivalent means I can substitute your long futures with a long CALL and short PUT at the same strike when you aren't looking at any time and you wouldn't notice the difference in behavior of the net position. No, he won't be whacked if vol jumps any more than he would be whacked if he had long futures and vol jumps. Again, for all intents and purposes, it's the same risk as being long futures, which isn't necessarily bad. It's impossible to make recommendations pertaining to what's a better position as the OP hasn't stated degree of bullishness and timeframe. AFAIK, the merits of the position weren't the subject of the thread. I think MTE, FullyArticulate, Daddy's Boy et al have now thrashed this topic to death LOL Prosperous trading! MoMoney.
One thing I noticed last week was that SPY calls were much more expensive than puts. With SPY at $138.00, Nov 138 calls cost $1.50 but Nov 138 puts only cost $1.20. So if you bought a synthetic long, it'd cost .40 or so and I suspect it would have nonzero theta.
Compare that to buying 100 SPY shares and not earning interest on 13,800 for the next few weeks. Calls are always more expensive than puts for that reason. You're only tying up $150 rather than nearly $14,000. This is the cost of carry. Risk Free rate is 5.4% (Eurodollars). I'm assuming you saw the 138 price on 11/1, which would be 17 days before expiration: 13,800 * .054 * 17/365 = $34.70 That's pretty close to the price you quoted for the synthetic long: $150-$120 = $30 So, there is "theta", but only inasmuch as covers the cost of carry.