Bica is a nonsteroidal antiandrogen while spironolactone is a steroid.
Bica is a superior AR antagonist with similar efficacy as flutamide (one of the stronger AA's and more successful in hair loss studies), but without the dangerous side effect profile of flutamide. Essentially bica blocks androgenic activity more selectively and effectively.
spironolactone, being a steroid, binds to other hormone receptors, including androgens. However it is non selective and binds to other hormone receptors as well, one of which can result in higher cortisol levels.
Also, spironolactone was created first and foremost as a potassium sparing diuretic (with AA side effects), so you pee a bunch and can't eat 30 bananas at once.