That's a nice question.
First of all, it is important to note that whenever a question of which drug to use arises, a number of factors should be taken into consideration, that is to say: prescribing a treatment is not simply a game of matching condition to drug (I am not saying that this is what the OP is saying or implying, this is just to illustrate that the process is complicated).
The physician should take into account the patient's main complaint (current condition), age, weight, sex (particularly important for hair loss...), sensitivities (if any are known), physical condition, underlying diseases (if any), other concurrent drugs, and more. These are all "filters" of a sort that help the physician narrow down the list of possible drugs that may be suitable for the patient's current predicament (this question deals with two therapeutic options, but sometimes there are many more drugs to choose from, e.g. when starting treatment for hypertension).
In addition, doctors also apply more subjective considerations to their decisions, such as their good judgement and experience, knowledge of drug prices (even if this is not a pure medical consideration, the doctor may apply it if he thinks that it's better for a patient to take a slightly less effective drug which is cheaper and the patient will be able to afford it, than if the patient will not be taking any treatment because the better option is more expensive/unavailable to the patient), and more.
Now, on the topic of this question:
Your comparison between minoxidil and finasteride is quite on-spot. You could also say that minoxidil is even more available on account that is is an over-the-counter drug, while finasteride requires a doctor's prescription.
In that case, minoxidil would surely be preferred by most clinicians. The cases when finasteride would be preferable to minoxidil are more limited and may include the following:
- Hypersensitivity to minoxidil (as with any drug, probably rare but should be borne in mind). Even susceptibility for non-dangerous side effects may cause a patient to avoid using minoxidil.
- Minoxidil was tried and found to be ineffective - this is a common issue in drug therapy, and for that reason it is good to have first-line therapy, second-line therapy, third-line therapy and so on.
Also, according to a publication of the AAFP (American Academy of Family Physicians): "Finasteride (Propecia), 1 mg per day orally, is approved to treat androgenetic alopecia in men for whom topical minoxidil has been ineffective."
- Finasteride is available as a topical solution but also as a tablet. Minoxidil tablets are not used for treatment of hair loss, only for treatment of hypertension. Some patients may prefer to conveniently take a finasteride tablet than apply/rub a minoxidil solution/foam on their head, particularly if they are in a an environment where it would be less convenient to use the topical form.
In conclusion, the selection of a treatment is based on both the doctor's and patient's preferences. These may not always be the same, but the final selection should be some average of both, so even when one alternative seems to shine above all others, it may not always be that way for all patients.