Spell Check will always tell you Saviour is misspelled, every single time you type Saviour and right click, it will give you Savior.
Webster , Yahoo and the average web search will give you never ending explanations and arguments as to what the difference is, which is correct and why the whole argument is a waste of time or of eternal significance.
Tradition seems to lean in favor of Saviour. i‘m kind of a traditional girl.
Some feel the u sets Saviour apart from the general many saviors of many things and refers to our one and only Saviour of the world. That’s kind of cute . Perhaps based in the 1600’s Kings James preference of including the u.
i choose to use Saviour when referring to my Lord and Saviour , the one and only, who gave his very life that i may have new life.
i use savior when referring to Jack from the Titanic who, “saved Rose in every way a person can be saved “.
It may be a quirk, it may be reverence and respect. What ever the reasons, i choose to use both, don’t allow it to confuse you or bother you, i will deal with the narrow mindedness of spell-check. You should see all the red underlines my auto editor is giving me right now .
Pappa P.S. Spelled the old way it contains the letters I.O.U. – Definitely a reminder of the I.O.U. for one life that was torn up and thrown away at the cross.