Another consideration - I love the combination of a 40 on the rear, with a 35 on the front of my Crosstrail - not knowing what the Sirrus will accomodate. I wouldn't be afraid to try a 32 on the front, either; but, the 35 on the rear made my bike ride much more harsh - the 35 on the front, didn't affect the ride, but sure helped the handling
The 40 on the rear helps the ride comfort, and the 35 on the front helps the steering and handling. You may be happy with a combination, as well.