Been to all 3 ballparks in last few years.
Always try to get field level seats (though this can be v expensive at Yankee Stadium)
Didn't think too much of the view from the nosebleeds at either Yankee Stadium or Citifield.
Excellent eats at Citifield include the nachos - they were awesome.
Personally if in Philly you'be gotta have a cheesesteak, Pats King of Steaks has a concession stand at Citizens Bank and if you can't get to Reading Terminal Market to experience the real thing - then this is a close second.
Yankee stadium - didn't eat as went out for big meal before the game - but beers weren't cheap - but they never are in such places.
As always - I'd recommend subway to and from all games (in Philly it's the end of the Orange line) and both ballparks in NYC are well served with subway stations
I'm sure you'll have a total blast - can't recommend any of these highly enough!