Is a Discordian at the same time also a Subgenius?
Nope, they're both "opt in" tribes. Being in one doesn't mean you're in the other.
Or are there Discordians who dont accept the Subgenius philosophy at all?
speaking as a mother,
I borrow a bit from the subgenius philosophy (I find the SLACK concept to be really useful), but I find most of it to be self-important crap designed to puff up your ego and make you feel like you're in the "higher" tribe.
What are the similarities and differences?
that's a pretty involved question I'm not qualified to answer!
A good place to start would be to look at the differences between Kerry Thornley / Greg Hill and Ivan Stang.
Kerry and Greg were zen students, and dress their ideas in eastern clothes (like the sacred chao)
Stang wears a mad scientist costume, and a lot of his ideas wear the capitalist-cult costume (like having to buy membership into the CotSG)
I feel attracted to Eris.
Careful -- that bitch is on a TEAR this year.