Using an SSD for backups is often the wrong choice.
SSD data retention is typically worse than an HDD. So if you want to go back and get your data at some point (which is, after all, the whole point of this exercise), the older the backup, the more likely it is that the SSD will have bad data and the HDD won't. Of course that's not the only consideration. If the backup unit is susceptible to being jostled, especially while in use, the SSD may still be a better bet. Other environmental factor may be relevant too (extremely loud noises, for example, can mess with HDDs).
Personally, I back up onto two rotating SSDs and two HDDs. Plus cloud sync.