Get the A300, we have one and we love it. Ours is a 2005 model. Yes its expensive (about $10000 more than the S300 just for the all wheel steer feature) but it is well worth the extra $$$. You wont rip up lawns thus saving you money and you wont rip up driveways. Also our machine still has the original tires on it soooo, how much does a new set of tires cost? If you run a normal skid a lot on tar you will probably have to replace the tires every year, maybe year and a half. Not with the A300.
Best part about the A300 is by the flick of a switch you can just put it in to skid steer mode if need be. The machine has plenty of power and it has 2 speed allowing you to go 12mph. Great for plowing. I would suggest demoing one, you will love it. Also look in to the new A770 its the new M series replacement for the A300.