What seats are better at a baseball game, Mezzanine or Infield Upper Deck seats?

According to me i prefer the best field box seats , that are between the edge of the screen and the dugout, not matter either on the first or third base side, maybe four to seven rows up. High enough to see the whole field, close enough to see the players' facial expressions. But from your question i prefer mezzanine, or the front row of the upper deck, anywhere between the bases. Great elevation, bird's-eye view, not as close as the field boxes but great in its own way.

If its in the infield, Mezzanine is better depending how far up in the mezzanine the seats are.

I would generally prefer a mezz seat, but it depends on how far out the mezz seats are.

Also depends on the layout of the stadium.

Chimp sits in his car ('89 Corolla), as close to the ballpark as they'll allow him.

first row, second deck by first base

The mezzanine seats....!!

The closest to the field you can get the better.