I never played PSO; wasn't interested in playing online (my Dreamcast didn't even have Internet going into it, anyway), and I heard single-player was dreadfully dull.
Gamecube version is the one you play for offline, and I put in 600 something hours of pure offline into that one as I didn't have a adapter at the time. I have played the DC one yes, but the Gamecube version is superior in everywhere and has way more content and better graphics. Also multiplayer splitscreen.
So if you're going to ever play PSO, I suggest either the Gamecube version or Blueburst. Game is amazing.