My preference is to use the same tire front and rear, but I won't throw away a perfectly good tire. When I discarded a damaged 26x1.9 and replaced it with a 26x1.5, I couldn't see throwing out the still good 26x1.9. I kept the larger tire on the front. Three reasons. Loosing your grip on the road is more catastrophic for the front wheel compared to the rear. Second, more weight is on the rear tire. Putting the more efficient tire back there gives more benefit than the less loaded front. Third, emergency braking is done with the front wheel. I think braking benefits from the larger surface area.