Swing axle is easier and more authentic. IRS is better. Way better, on track or on the road.
Most guys just build what they get. If you have an IRS pan, do yourself a favor and get an IRS transaxle. A later model one is better; they're a little stronger, but like Danny says, any one will do fine in front of 95 percent of the engines you're likely to use if it's gone-through and rebuilt to the normal heavy duty standards.
If you have a swing axle chassis, build a swing axle car. Buy a camber compensator and bolt it on before you get to the road test phase. Swing axle cars handle OK with these, and if it's paired with a good front sway bar you should be able to stay out of trouble.
Now stand by for gear ratio advice. ...