I really don't think that the crop damage difference between the two is going to be as bad as some of these guys are saying.  I've sprayed using a pull type sprayer on our Cat 55 for a couple of years and once a guy gets some experience row cropping with one and learns how to turn on the ends crop damage isn't bad at all.  In mud the tracked tractor will spray through stuff you can't get a 4 wheeler across. 

Even when it's not muddy I like the compaction advantage to the tracks, people always complain about compaction caused by grain carts but then they will take a sp sprayer with just as high or higher ground pressure and drive it through their standing corn and think nothing of it. 

Operation costs are going to be lower with the tractor, with the tractor you can shift up and throttle back, in long rows I will spray in road get at 1,300 rpm and it just sips fuel then.  Generally repairs are going to be cheaper on the tractor and it won't depreciate in value as fast as the sprayer. 

This picture shows where I turned on the ends while spraying with our Cat on 18" belts in corn and a 90' boom.