Tricky question.
As someone who suffered from a nasty flu, as well as covid19, I'd stick with the flu.
During the first day of both illnesses, I can say for sure that the flu was worse, as the fever won't go down for days, plus hallucinations and all.... while the covid symptoms got a bit better as days passed... even after all that I'd stick with the flue, since I now feel the covid consequences, 2 months after recovering, more than I did during the illness itself. Constant suffering of muscle pain, cognitive problems, trouble breathing, loss of hair, not been able to smell anything, and that without mentioning the tinitus and the heart problems.
Nope. Flu it is.