IE800 is better in terms of soundstage, that's the biggest pro of IE800 though. In almost all other things I can think of, SM64 is better. It's musical, fatigue free, has no sibilance and has a full sound. IE800 tends to be too bright and sibilant at times and the sound is lean. Soundstage, yes that's the best thing about IE800. That said, SM64 is very good in terms of soundstage too, in fact it doesn't sound like an IEM at all. Higher end S-EM6 is probably better in terms of soundstage but I doubt they're as fun sounding as SM64. SM64 is probably the best upper mid range IEM out there and can give many higher end IEMs a run for their money.
Also, I've never heard such lovely mids from an IEM before...