HMG only has a couple hour half-life, just like the actual LH/FSH in your body. It needs to be injected once a day, preferably twice a day. (75iu 2x ED) hCG on the other hand has about a 3-4 day half-life so you only need to be injecting it every few days.
This makes HMG it pretty expensive, but if you can afford it, its far superior to hCG. (less estrogenic conversion, and less desensitization)