Gram per gram, HCL is “stronger”. That’s because there’s data showing HCL gets 60% better absorbed than monohydrate. Which means 10g of monohydrate equals 6g of HCL. Both in they’re respective proper doses are as effective. Monohydrate seems to cause some stomach discomfort in some, not seen with HCL. Also, with monohydrate, people experience water retention under the skin which give them a “softer” or “puffy” look. Not seem with HCL because of better absorption. HCL is also more expensive.