I think larger, lower-pressure tires resist some types of flats better than do skinny high-pressure tires.

Since rear tires carry more weight and have more flats, it makes sense to put the more flat resistant tire in back.

That said, I ride with 25mm tires, front and back.

I have, formerly, ridden with a Conti Gatorskin in back and a Conti 4 Seasons in front.

Conti doesn't have a Gatorskin on the market right now, since, it appears, that the current 4 Seasons matches the Gatorskin's flat-resistance.

I predict this Fall Conti will introduce a new Gatorskin with increased flat-resistance.

In the meantime, I ride with a 25mm 4 Seasons front and back, and I will switch to Conti's new Top Contact Winter in 28mm as soon as we have our first frost.

Riding with two different sizes of tires changes a bike's geometry, and thus handling, a small amount; but, then, even changing matched sizes of tires changes a bike's geometry, so that matched 23mm, 25mm and 28mm tires all create a different amount of trail at a given rake and head tube angle.