Not Medical Advice: Potassium phosphate is a generic term for the salts of potassium (K) & phosphate ions while potassium chloride is a metal halide salt composed of K & chlorine.
Potassium chloride is a white, odorless powder with a salty taste. Like other salts, it contains an alkali metal and a halide. While chemists can synthesize it in the lab, most of the potassium chloride you see in your daily life occurs naturally in foods or is dissolved in tap water.
Potassium chloride is indicated for the treatment of hypokalemia also known as low potassium levels. It is readily available and the least expensive, but it can cause stomach ache and interferes with vitamin B12 absorption. Also, chloride can contribute to the retention of sodium.
The potassium and sodium salt forms of phosphorus are called phosphates.
Potassium phosphate is readily absorbed, improves phosphate balance and has the advantage of decreasing "perceived exertion" during exercise. It is inexpensive and available in many stores. Potassium phosphate should not be taken with antacids since this will lessen absorption.