I'm not sure what you mean by "offroad". If lots of sand, the 57-559 (2.25") Extremes will be slightly better than the 55-559 (2.15") Mondials. If lots of rocks, the Mondials have much heavier sidewalls. Schwalbe width specs are to be taken with a grain of salt. However, if you actually look at inflated 57-559 Extremes versus inflated 55-559 Mondials, you will see the Extremes are significantly wider. The Extreme also has a more aggressive tread, so will run better on dirt but worse on pavement. Extreme is lighter weight and hence will accelerate faster.
I run into a mix of sand and rock and pavement. I definitely want all the sidewall protection I can get from those rocks, and that gives a big plus for the Mondials. When I hit true sand, the Extremes would not help over the Mondials. I'd be pushing in either case. For dirt, I tour with a full load and don't ride aggressively, so don't need good grip, so the tread on the Extremes is no advantage. For pavement, the Mondial is better. I don't care about rapid acceleration, just steady speed on the flats, and tire weight makes little difference when not accelerating. All things considered, Mondial 55-559 is best for me, mainly because of the sturdier sidewalls.
[edit to fix a type re specs]
