Prescription potassium citrate is extended release, which will be a factor in how it gets used in the body.
The highest dose OTC potassium is usually 99 mg elemental potassium, and contains who-knows-what-else for fillers.
Prescription potassium is always dosed in mEq, not mg, which makes it even trickier. 99 mg elemental seems to work out to about 2.5 mEq, so depending on your dose you might end up taking way more pills of OTC to get the dose you need, which could end up being more expensive, and certainly more hassle, then just taking the prescription.
Because of the inconsistencies in the OTC formulations, I recommend sticking with the prescription. There should be a generic for Urocit-K, which should help with cost. It's not the cheapest generic out there but way less than brand name.
Also, it can pay to shop around at different pharmacies. Often there are big differences in price between different ones.