When do you suggest to use "dimenticare" and when "scordare"? Is "dimenticare" only for reasonings or also valid for people?
Avevo dimenticato il tuo viso
Avevo scordato il tuo viso
Yes, they have the same meaning and both could be used in your examples.
Nonetheless, there is a little difference in the meaning due to their Latin origins.
That's why you can say ho dimenticato/scordato il tuo viso but you would usually say il primo amore non si scorda mai.
You can use them both interchangeably, they basically mean the same thing, although scordare may also be referred to an instrument that is out of tune.
Despite the fact that people believe scodare is less formal than dimenticare, it is not in fact exclusively part of any regional vocabulary and each region applies its own peculiarities to this verb when in use.
When I was a kid, my teacher would correct "scordare" by saying "you are not a guitar". The joke was that "scordare" sounds like the opposite of "accordare" (to tune). Because of this early imprinting, I tend to use "dimenticare" but I noticed that "scordare" is being used more and more in everyday Italian.