SPX and SPY have about the same relative performance, whereby SPY is about 1/10 of SPX.
The main differences are:
American Style (SPX) vs European Style (SPY)
Cash Settled (SPX) vs Physical Delivery (SPY)
Here's much more info about the differences between SPX and SPY: https://www.projectfinance.com/spx-vs-spy/
It says "For settlement type, SPX appears to be the clear winner in the eyes of most traders". Hmm. I don't think so.
I think you mean the Early Assignment risk with American Style when you are short the options. In this case the position gets closed and you have no chance for rebounds. But Early Assignment is not possible with European Style, ie. in case of SPY.
May I object?
SPX is European style, is cash settled, no early exercise.
SPY is American style, physically settled, exercise possible anytime if option ITM.
Or am I wrong?
Kind regards
Ive backtested it and know its wrong or optimized