Considering all of the zero-turn mowers I've seen mowing lawns with a perpetually flat front tire, I would consider an airless tire: they never go flat, the ride is very comparable to pneumatic tires, and they generally last a very long time. There is a company called Amerityre, that makes a closed-cell polyurethane tire that won't ever go flat, wears quite well, and rides nice. I believe its biggest drawback is that they're pricey, but considering the downtime constantly diddling with flat front tires, it should pay for itself in a few years. Plus, it's American made, which you can't say about nearly all of the current pneumatic tires on the market.