Humalog 75/25 contains insulin lispro (rapid) and insulin lispro protamine (intermediate).
Humulin 70/30 contains insulin isophane ( intermediate) and insulin regular (rapid).
They are both said to work within 10 to 20 minutes after injection, peak in 2 hours, and keep working for up to 24 hours. So, as far as I can see the difference is that they are slightly differently structured insulin molecules. In theory they should react similarly, in practice one may be better for each individual.
There must be some difference, since otherwise one manufacturing company wouldn't make two forms of a drug that behave identically. Maybe call Lilly if you're really interested.
Sorry I can't help more.