An answer to your question online backup is the new form of backups nowadays.
To mention a few of it's pros you may be able to get to your files via the internet which can be very handy. Plus you are protected if your offline backup is lost, stolen, or destroyed (fire/flood/malfunction/etc).
However, you wouldn't have direct control over your data and if your provider goes out of business, what happens to your data?
As for my backup plan i use both to be honest i use backblaze to backup my pics and files to the cloud. And Genie timeline as a local backup software with a disaster recovery plan to make sure all my programs would be restored in case of hard drive failure.
finally i wouldn't recommend a flash drive to use.
"PDR reports a 300 percent year-on-year increase in cases of USB drives that have "suddenly stopped working," as a result of "faults, misuse and an increasing number of poorly manufactured devices on the market."
-According to a report issued by data recovery specialist Payam Data Recovery (PDR), cases of faulty USB flash drives are on the rise, growing at an even faster rate than unit sales themselves.