Depends on which you prefer, I've heard that Tales of Symphonia has a better battle system, but lesser story, and Star Ocean has great story, and a great battle system I might add, but ToS' is better.

I mean, they are pretty similar games, always have been since the first Star Ocean and first Tales game, the teams always competing to outdo each other, made for some great games.

Personally, I'd say Symphonia, then Star Ocean.

The only reason I got Star Ocean first is because Symphonia isn't out here in the UK until November =(