Well, if you're looking for online action, certainly you won't get much with Okami...
PSU has so far been getting excellent reception and has broader gameplay options, but Okami is simply a unique, must-not-miss
experience. Not to mention, IMHO, flying straight to the top (certainly AMONG the top) gameplay-wise for its' genre.
Okami has more "RPG elements" than resembling an RPG like PSU does, but it flies high on the "fast-paced battle system" part.
What you describe that you
want fits more to PSU, but for a self-contaned weekend of just plain enjoyable gaming...? You'd be able to play through Okami (I'd estimate playtime at ~30 hours, adjusted by how much you like messing around and hunting things down. Hehe...) and take in the whole experience in one go, which (after spreading my gameplay over a few weeks) I think has much appeal.
PSU is, of course, going to be a more open-ended, extended experience. You might still want to play Okami after you finish the plotline for "collecting everything" purposes, but PSU will keep offering you new fun and new people to meet down the line for lasting appeal. IMHO, Okami fits the bill more for a "weekend of gaming" and PSU for "some time this weekend, then next, then whenever I'm bored during the week..."
Personally, I think you should hedge your bets. Get both, play Okami first, and if you still have time by Tuesday, make inroads on Phantasy Star! Hehe...