Nickel works in almost every worship setting. Stainless steel gives more note definition and attack, which suits modern worship bands and larger productions. Flatwounds suit traditional settings, and semi-flats reduce finger noise while keeping much of a roundwound's clarity.
For most 4-string basses, .045–.105 is the standard recommendation. For 5-string, .045–.125 gives a balanced feel and a clear, supportive low B. Many worship bassists play 5-string for songs in lower keys, but a 4-string is still an excellent choice.
Many players put fresh strings on before Christmas, Easter, conferences or recordings. You don't need separate strings for contemporary and traditional services.
S.I.T. options: Power Wound Nickel for balanced versatility, Foundations Nickel or Fusion Wound Nickel for a more modern or premium nickel, and Foundations Stainless or Fusion Wound Stainless for extra articulation.