I would go for Infinite Undiscovery, as Star Ocean 4 is still a while ago, though they are very similar in terms of their battle system, except IU is closer to an MMO (more like XII) and they are both made by Tri-ace. I don't really know about the story, though IU does have the issue of a ridiculous title... though I am sure you can get over that.
The only issue with the SO battle system is that it can turn into button mashing, though that is only my experience from a small part of 3 (I still haven't got around to finishing it since getting it in April)







