What is best Spironolactone or Finasteride for females?

Head to head comparative studies have not been done. For younger pre-menopausal women it is important to understand that Spironolactone is used more often by dermatologists (and hair physicians) than finasteride given the serious harm to a baby were that woman to become pregnant. Both are not formally FDA approved and both can harm a fetus during pregnancy. Some physicians shy away completely from prescribing finasteride on account of this issue and will not discuss its use in women period. But this is a highly practice and physician dependent decision. Taken together, we don't really know if finasteride or spironolactone is more effective. What is important here is that both have undeniable evidence of benefit in a proportion of women with androgenetic alopecia. Strict contraception is needed when either are used.

Female pattern hair loss or genetic alopecia is the most common reason of hair loss in women and it is more common than it is thought to be. 25% of the women at the ages of 35-40, 50% of the women at the ages above 40 suffer from hair loss. 20% of the women with hair loss have a positive relative who suffered/is suffering from hair loss. Other reasons of hair loss are: Hypo- and hyper-tiroidism (hair loss may be one of the early signs of thyroid dysfunction, and hair loss stops after treatment); polycystic ovarian syndrome (an inherited ovarian disease, seen in 10% of the women during adulthood); pregnancy-related hormonal changes (temporary hair loss which can be seen during pregnancy stops after birth)..

The reason of the hair loss may related with factors other than genetic like connective tissue disorders, stress, anemia, lupus, medications, hormonal and seasonal changes, nutritional problems, severe diets, bulimia, protein/calorie deficiency, zinc and essential amino-acid deficiency, mal-absorption (intestinal and digestive problems), A-vitamin excess, general anesthesia, affective mood disorders.
So you may need to have a full exam with an hair surgeon, internalist and/or endocrinologist to find out any reason for hair loss.

There is not cut and dry answer to your question. Finasteride is not good for 30 year old females, but spirinolactones may be appropriate