To those who feel Jewish messianism was the home soil of the new faith then the form Christos (word coined as rough equivilant of Hebrew Messiah) would be assumed the original with Chrestus being a corruption. Interstingly Gnostic Christains felt the two terms were grades of initiation, the newbie convert called Chrestus until reaching full intiation and annointing then called Christus. They (broad brush) felt Jesus was not the only Christus but all could attain to this Christus state. ( Makes me think of Paul.)
I wouldn't say one had to be the original - there are two considerations - they can simply be parallel notions of the time
Interestingly the gnostic understanding isn't too far removed from the idea of baptism with water, and spirit - but it seems that there may have been men involved judging an orderley second step (accurate knowledge? lol)
Regarding "christian" - if one follows Russell, one is a Russellite - if one follows the annointed (christus), they are christian - they may be annointed too, or a son, as He is a son etc.