We are going to focus on 2 main hormones here. Leutinizing Hormone (LH) and Follicle Stimulating Hormone (FSH). Both hormones are released from the Pituitary Gland in the brain and stimulate the testicles to do different things.

LH – Stimulates the Leydig Cells in the Testes to produce testosterone
FSH – Stimulates the Sertoli Cells in the Testes to produce sperm.
***Both of these functions need to occur for sperm production. LH needs to provide an environment rich in Testosterone for sperm production to occur.

See diagrams below:

So what does HCG do?
HCG simply mimics LH to signal the Testes to produce testosterone. HCG is a hormone who’s molecular shape is structurally similar to LH therefore it fits in the same receptors on the Leydig cells.

What does HMG do?
HMG is a combination/mixture of ACTUAL LH AND FSH. So HMG stimulates both the Leydig cells AND the Sertoli cells to produce testosterone and sperm respectively. No mimic hormones like HCG, these are the real things.

Conclusion
HMG is superior especially from a fertility standpoint. It performs the same function as HCG (or LH) AND it performs the functions of FSH, both which are needed for adequate sperm production.