Sodium and potassium ion concentrations are important in membrane transport of charged particles in and out of cells (e.g. proteins, fats). It is critical in creating ion currents in heart cells. Low potassium could lead to heart problems such an irregular beat, or even fibrilation. Frequently in dogs high potassium is seen in such diseases as kidney failure, or diabetes. Low potassium seems to be rare.
If it were my dog I'd go with the Therea K first, and then work with the Vet to see if one could alter the diet with the goal of reducing and later eliminating the prescription supplement. But you should get the potassium levels back to normal as quickly as possible. You should discuss or ask the Vet about possible underlying causes, and how the level is to be monitored initially.