The SHO was available with a performance package that includes a deeper axle ratio, revised suspension and better brake pads. MkSs have never had that option. Opinions vary as to it's performance benefit. SHOs without the performance package are mechanically identical to the MKS ecoboost. The MKS is a tad heavier primarily due to the larger moonroof and a few other things. Performance wise, stock or tuned, there is virtually no difference.
My MKS would run 0-60 in about 5.3 seconds stock and 4.09 seconds with an LME 3bar tune. I never ran the quarter mile stock but magazine road tests put it at about 13.7 or so. Tuned, mine has run the quarter in 12.8 but that was not under optimal weather conditions and I don't launch the car very hard.
I would be more interested in the car's condition, mileage and how it is equipped than whether it is an MKS or SHO. Of course, your personal preference regarding appearance, how much emphasis you place on little luxury touches, etc. would also be a factor. While not always the case, typically I would expect the MKS to have been driven by a bit older and perhaps less aggressive driver than the SHO so the MKS might be less likely to have been abused. No guarantees on that, of course!
