My Specialized Crosstrail came stock with 45s. I happened on good deals on a Schwalbe Marathon Supreme in the 40, and bought 3. I then ran across a really good deal on a set of the same tire in 35, so bought 2.

Since I had all these tires, I started mixing and matching.

I can't tell the difference in ride or handling between the 40s and the 45s. A 35 0n the rear is noticeably more harsh.

The 40 on the rear doesn't have any effect on handling, meaning it's as good as the 35.

The 45s, are cushy, (same as 40) but handling suffers.

However, the 35 on the front rides the same as the 40 or 45, but appreciably improves steering. Quicker and more precise.

What I ended up with, are 35 on the front, and 40 on the rear, and absolutely love this mix,

Don't be afraid to mix and match sizes.

I used to get a flat every 100 miles with the old Specialized Burroughs, and haven't gotten a flat in almost 4 years with the Supremes, and they are nowhere near as heavy as the Marathons you are looking at.