It's annoying that you use brand names.
Dutastride is better. (Avodart Brand name)

Medical trials.
0.5mg is normal dosage for dutastride Finrastride provides 75% DHT blocking potential.
Dutastride provides nearer 93% because it irriveribly deactivates both Alpha 5 reductase enzyme isotypes where as Finristride will only deactivate 1 isotype.
Spironolactone blocks the bio-synthesis of Testosersone itself rather than DHT biosynthisis direct not only interfereing with DHT levels but also interfering with the resivour in whitch DHT is formed out of. Thus reducing the level of circulating by several orders especally if combined with one of the other above mentioned drugs. (think 99.8% DHT reduction)

Thus taking Spironolactone and Dutastride together would provide a synergistic extreme level of Androgen inhibition based on the options you've provided.
Be aware such an extreme level of androgen inhibition would cause feminisation so just letting you know if your only goal was preventing hair loss.