* Σωτηρία (Sōtēria): This is probably the closest and most common way. It's the standard spelling of "Salvation" in Greek, and shares a similar sound.
* Σωτιέρα (Sōtiēra): This adds an extra "ε" (e) to the ending, which might better reflect some pronunciations of "Sotiere". However, it's less likely to be readily understood by a Greek speaker unless the context is clear.
Ultimately, the best spelling will depend on context. If you're using it as a name, the first option is likely safer and more easily recognized.