The best approach may be to report on the results of geographic studies. We know that centers and hospitals where a high number of total knee replacements (TKRs) are done have better results. The same is true for high-volume surgeons.
It may also depend on whether or not she is planning to have both the knees replaced at the same time (simultaneous) or through a staged procedure (first one, then the other).
In general, more surgeons in the northeast do simultaneous TKRs. And more high-volume surgeons are at these centers. With only one anesthesia and one hospitalization, costs are much less for the simultaneous procedure.
If your mother has no (or very low) out-of-pocket expenses, then this may not be a direct selling point for her. We all have a responsibility to keep health care costs down, so you may be able to appeal to her from this point of view.
After care and rehab are important factors in the final result of this operation. If you can provide in-home care, transportation, and social interaction, then you may want to emphasize these benefits to her as well.