The best rollover protection advice I can give is to buy a tractor with a low center of gravity, and then operate it safely (avoid driving up or across steep slopes, slow down when turning, never turn sharply with a raised front end loader, and never hitch anything to the tractor other than at the drawbar).

In terms of the tractor's basic geometry and configuration, the biggest factors in determining its resistance to rolling over are the center of gravity and the rear wheel spacing.  The lower the center of gravity, and the wider the rear wheel spacing, the less likely it is to roll over.

The configuration of the front end matters, but not nearly as much as the center of gravity and the rear wheel spacing.  A "wide front end" is not rigid like the tractor's rear axle--it's designed to rotate about the long axis of the tractor, up to a point, so that the tractor can drive over uneven terrain and still keep both front wheels on the ground.  Until a tractor tips far enough that the wide front end rotates all the way to its stopping point, the wide front end doesn't provide any more resistance to overturning than does a narrow tricycle configuration.  My point isn't that narrow front ends are safe--rather, it's that wide front ends don't make a high-center-of-gravity tractor safe either.  If you're buying a tractor to use on hilly terrain, choose a utility tractor for its lower center of gravity.

If rollover is a real concern for you, consider buying a tractor with a professionally-installed rollover protection system (ROPS).  That pretty much rules out any tractor built before the early 60s or so, when ROPS first became available and when tractors began to be designed for them.