"Nunca" refiere a algo que no haces en tu vida diaria, pero que podrías llegar a hacer. "Jamás" es más radical; refiere a algo que nunca en tu vida harías. Nunca veo televisión (I never watch TV) Jamás usaré esos zapatos (I will never wear those shoes... ever!)

Tried writing my own explanation but found this to say it better than I did: ‘’’ "jamas" is more radical, it's a decision, something that probably will never happen. e.g. "Yo jamas ire a esa casa"/ I will never go to that house. "Ella jamas sera tu esposa"/ She will never be your wife. "nunca" is equivalent to never, something that has not been happening up to the present time e.g. " Yo nunca como cebollas"/ I never eat onions. "Nunca tomo siestas por la tarde"/ I never take naps in the afternoon. ‘’’

Puedes usar ambas / You can use both and the meaning is similar :)

Son similares puedes usarlas las dos