A swing axle car can be modified to handle well and it takes the usual mods to get it there; 3/8" - 3/4" front anti-sway bar, stiff rear camber compensator, stiffer torsion bars, stiffer shocks, wider wheels (which will probably need to be custom because of the limited space you have to work with) and, as important as anything, wider, 40-series low-profile tires. Do all that, and it will hard-corner at about 75% of the ability of an IRS car that has been modified with 3/4" anti-sway bars front and rear (camber compensators aren't used on an IRS car), stiffer shocks, stock torsion bars and the same wheels and tires. BOTH of those cars will ride stiffer than stock - just the nature of the beast.
The biggest difference is always seen at very hard cornering limits - The geometry of the swing arm rear is such that the inside rear tire on any corner is going to try to tuck under the car as the inside of car lifts because of cornering G-force. As the side of the car goes up, the axle tucks the wheel under the car and the tire rides up onto the sidewall, losing traction.
On the other side of the car, the G-force is pushing the outside wheel up into the wheel well while applying lots of lateral force to the bottom 1/4 of the tire, causing it to roll up onto the sidewall and lose traction (sound familiar?). All of this can be felt in the steering wheel and in your butt as it begins, but the transition from weakening traction to no traction happens in an instant with almost no warning. The technical term is a "transition from understeer to oversteer", meaning that you go from the front end plowing sort-of straight ahead as you turn the steering wheel more and more to the side (understeer), to the rear end losing traction and letting go and coming around to meet the front (oversteer). It ain't pretty and can cause excessive staining of your pantaloons. Also, because of that swing-arm geometry there is also a tendency of the car to roll up onto the outside sidewalls of both front and rear wheels, lose road grip and allow the car to roll over. Your day just went from bad to worse.
IRS can do the understeer/oversteer thing, too, but it takes a hell of a lot more G-force to make it happen. That's why swing arm cars often have trouble keeping up with "spirited" and well-set-up IRS cars. IRS cars, by the nature of the suspension, corner flatter with more of the rear tire rubber on the road at all times because the wheel is only allowed to go straight up and down, not swing under as the swing arm cars do, so it takes a lot more force to push laterally onto the tire sidewalls (and you can feel the transition to the limit of the sidewall in your butt). You can still feel the under/over transition as it begins to happen, but it is slower and actually more controllable such that the car can be steered with the throttle foot by applying more (to induce oversteer) or less (to allow understeer) throttle and/or hold the car right on the brink of oversteer for the fastest line through the corner. IRS cars are much harder to cause to roll over. Possible, yes, just much harder to induce.
So, all that said, so you understand the differences, we get to Troy's last paragraph:
"If you're just a pleasure cruiser and never plan to try to keep up with Robert or Teby, just get a swing axle car and spend about $100 bucks on a camber compensator. Since you are getting a Pat Downs motor, you're probably not just going to be a pleasure cruiser, so get the IRS. "