Here is a great article on when to use the StringComparison.
DO: Use StringComparison.Ordinal or OrdinalIgnoreCase for comparisons as your safe default for culture-agnostic string matching.
DO: Use StringComparison.Ordinal and OrdinalIgnoreCase comparisons for increased speed.
DO: Use StringComparison.CurrentCulture-based string operations when displaying the output to the user.
DO: Switch current use of string operations based on the invariant culture to use the non-linguistic StringComparison.Ordinal or StringComparison.OrdinalIgnoreCase when the comparison is linguistically irrelevant (symbolic, for example).
DO: Use ToUpperInvariant rather than ToLowerInvariant when normalizing strings for comparison.
DON'T: Use overloads for string operations that don't explicitly or implicitly specify the string comparison mechanism.
DON'T: Use StringComparison.InvariantCulture-based string operations in most cases; one of the few exceptions would be persisting linguistically meaningful but culturally-agnostic data.