10 SPY 130/132 @ 0.50
Return @ Expiry (net commiss.)= $500 - 25 = $475
ROM 31.7%
1 SPX 1300/1320 @ 0.40
Return @ Expiry = $400 - 2.5 = $397.5
ROM 24.8%
1 SPX 1300/1320 @ 0.45
Return @ Expiry = $450 - 2.5 = $447.5
ROM 28.9%
If we assume round trip then SPY has less advantage. Assume the short is bought back at 0.05 in both cases and not held to expiry.
10 SPY @ 0.50 = $450 - 37.5 = 412.5
ROM 27.5%
1 SPX @ 0.40 = $395 - 3.75 = 391.25
ROM 24.5%
1 SPX @ 0.45 = $445 - 3.75 = 441.25
ROM 28.5%
So if we assume an early exit and a better fill on the SPX, then SPX is slightly better. But, then we have to consider the idea that SPY technically don't qualify for 1256 treatment. (I'm not looking for an arguement on that point.)
Held for worthless expiry after tax (approx 27% tax rate):
SPY 0.50= (0.73)*(475) = $346.75
SPX @ 0.40= (0.6*0.85*397.5)+(0.4*0.73*397.5)= $318.80
SPX @ 0.45= (0.6*0.85*447.5)+(0.4*0.73*447.5)= $358.9
So if we make some simple assumptions we can come to the following conclusions:
If you are going to close out early then SPX is probably better. If you plan on holding through expiry, then a crappy fill on SPX makes SPY the better choice even though you lose most of the advantage to taxes. My argument has always been that taxes aren't paid immediately so you should do what makes the most money right now, as that leads to better compounding up until tax time.