There really aren't many more electric accessories on a Cadillac than the average late model car. Most (not all) of those that are present aren't terribly expensive to repair/replace. The STS has fewer digital instruments than the SLS, thus one less potential headache. The only potential $$$ on the Cadillac (vs. Regal) would be the Northstar engine, since it is a lower-production and more complex powerplant than the 3800 V6. Needless to say, complete service records would be a huge plus for the Cadillac.
An afterthought: Does the STS have a load-leveling rear suspension and/or electronic shock valving? Potential $$$.