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.
Writing down a seed still requires some kind of computer-like device to generate the seed. If the seed was generated on a HW wallet, then perhaps storing a seed on a USB drive would require an additional computer. However, if you remove that assumption, using a USB drive requires no additional security measures above using a paper backup.
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 or import them later do insecurely because they far harder to create and use than a simple software wallet.
This is not true. I am assuming you are referring to a wallet that is not a hardware wallet.
If you compare the potential security risks associated with creating (and using) a paper wallet, and a wallet stored on an encrypted hard drive or USB drive, using a paper wallet would have all the security vulnerabilities associated with a seed stored on an encrypted hard drive or USB drive, and would also have additional security vulnerabilities.
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.
I will advise people to follow the 3-2-1 rule:
3 backups using
2 mediums of storage, with at least
1 backup stored off-site
When comparing your two choices, you are changing two variables, the number of mediums of storage and the distance each backup is located from the other. The variable that increases the benefit in your comparison is the distance between backups, not using a single medium of storage.