In the past we had a John Deere 9520 four wheel drive tractor. In order to make sure we get field work done faster we got a new plow. As it turns out the new plow we purchased is a little to large for the old tractor. We are now selling the old one are are looking at either a John Deere 9620RX or a Case IH 600 quadtrac.
It is generally more expensive to fix a John Deere but it is less likely to break down than the Case IH quadtrac. They can have transmission problems as they might not be able to handle the amount of work that it takes to turn the tracks and such. John Deere has a selection of warranties that one can get for their tractor as does Case IH.