difference between bitter orange peel and spanish orange peel?
Not knowing the exact description in the catalogues, or the use for which they're intended, it's difficult to know if there is any difference. I guess that the Spanish peel will be Valencia oranges which is pretty sharp - the orange is briefly available in the UK each year and would be the normal one to use for making marmalade, for example.
I think perhaps you are thinking of Seville oranges? They are the classic marmalade orange, also known as "sour oranges" and common in some Latin American and Caribbean cuisines as well. Valencia is the common name in the US for juice oranges (as opposed to eating oranges, navels).