I'd say Philly; only 'cause you haven't been there before. Camden Yards is better, because it was built with town around it; not just a distant glimpse of Center City out well beyond the outfield, which you get with the new Vet.
The thing is, I think there's only standing-room only tickets left in Philly to see the infidels in pinstripes; so head down 83 to Balmer. And you can park for $10 at the hotels by the Inner Harbor and it's a 10-minute walk, 15 minutes if you take it really slow or have children with you. If you've got time to hang out and make a day of it, the vote is easily for Baltimore just for all you can do around the Inner Harbor or Fells Point.
Back when the O's were worth watching, parking was much worse and the hassle of Light Rail mighta been worth it. But now that they blow - and always will as long as Angelos and his meddling sons act like Ted Turner of the '70s - tickets and parking are plentiful. So hit Baltimore tomorrow and take the next opportunity to head to Philly.