If you are comparing storing a backup using medium A and medium B, in order to compare the two mediums, you need to assume the same security measures are taken, unless doing so would not be possible.
If you are comparing which storage medium is more secure once they have been created, then sure, you assume they are both created securely. But in reality, that is not the case. Almost anyone can write down a seed phrase securely - just make sure you do it with nobody else around and no cameras or webcams pointed at what you are doing. Very few people on the other hand can properly create an encrypted USB drive back up from a properly airgapped computer, leaving no traces of what they have done and leaking no information in the process. This is an important point to consider.
Just as if we compare a properly created paper wallet to a properly created software wallet, then the paper wallet is exponentially more secure. However, we know from experience that many people who create paper wallets to do insecurely because they far harder to create and a simple software wallet.
Every medium of storage has the potential for data loss under certain circumstances, and the only way to mitigate this risk is to use multiple storage mediums.
This is also mitigated by using multiple storage locations, not just mediums. If the circumstance is, for example, all my paper wallets are vulnerable to fire or water damage (which is no different to a USB drive), then I am far safer storing two paper copies in two physical locations in different states than I am storing a paper and a USB copy a few blocks from each other.