Based on what I have seen come across the auction block over the last few years, I'd say that the majority of Sterlings, Sebrings, and Cimbrias have IRS. Every once in a while you see one with a swing axle, but not as often as you see IRS.
Common consensus is that the swing axle transaxle is a little bit stronger (and is often used in drag cars) whereas the IRS set-up is better for handling.
For all practical matters, they are essentially equal for our cars as long as they are set up correctly. Handling on the swing axle cars can be improved with a fairly simple, bolt-on camber compensator. For both set-ups, the rear suspension often should technically be lowered a little from stock to compensate for the fact that our cars are significantly lighter than the stock Bug and therefore tend to sit a little high which can throw off the rear geometry a little.