Quote from ktm:
Margin wise, yes it is a huge advantage as you get about twice as much margin if you are on BOTH sides of the market. The simplest example is an OTM naked call and an OTM naked put: with SPX you pay separate margin for each, with SPAN you esssentially only pay for one side. It has been a few years since I really looked at SPX as I've been using SP/ES for that time, although I understand SPX has 60/40 tax treatment nowadays.
I think liquidity is far better in SPX, although the CME recently moved the dime tick theshhold to $5 from $3 and added a second set of contracts to expire EOM, so there's more rolling and writing capability throught out the year. It is still very easy to move 400 contract blocks in both SP/ES.
An additional concern for those managing OPM is that SPX is considered a security and subject to SEC regs, whereas futures options fall under the commodity regs of the CFTC (NFA). There are very distinct differences in licensing and registration requirements at the state and national level depending on whether one trades securites or commodities.
Another advantage of SPAN is the ability to incorporate futures into the position and reduce exposure and margin. If you are going with spreads, as most are in these products, you need the ability to enter spreads. Legging is tough over in the SP/ES camp!!!