This paper compares approaches to music key detection based on popular key-profiles with a new key detection method that utilizes the concept of the signature of fifths. The signature of fifths is a geometrical music harmonic-content descriptor. Depending on the scenario, it may reflect either the multiplicities of occurrences or the aggregated durations of individual pitch-classes of the chromatic scale in a given fragment of music. In this study, we compare the efficacies of a few strictly correlational key recognition approaches based on music key-profiles (i.e., Krumhansl–Kessler, Temperley, and Albrecht–Shanahan) with a new method that implements the concept of the signature of fifths. All the experiments were performed in a collection of music pieces comprised of preludes by J. S. Bach (The Well-Tempered Clavier-Book I), preludes by F. Chopin (Op. 28), and songs by The Beatles (from the album A Hard Day’s Night). In the scenario implementing the aggregate durations of individual pitch-classes, based on the analysis of the shortest initial fragments of music for which the key was indicated in all the considered approaches, the key detection efficacy obtained with the method using the signature of fifths was greater than the efficacies obtained with the strictly correlational approaches utilizing key-profiles (on average by 9.27 pp). In the case of the analogous analysis carried out for the scenario implementing the multiplicities of occurrences of individual pitch-classes, on average, greater efficacy was observed for the strictly correlational approaches based on key-profiles (by 2.7 pp). The conducted experiments confirmed the new key-detecting method offers advantages in computational simplicity, stability of decision making, and the ability to successfully determine the key based on a very short fragment of music.