FXCop typically prefers OrdinalIgnoreCase. But your requirements may vary.
For English there is very little difference. It is when you wander into languages that have different written language constructs that this becomes an issue. I am not experienced enough to give you more than that.
OrdinalIgnoreCase
The StringComparer returned by the
OrdinalIgnoreCase property treats
the characters in the strings to
compare as if they were converted
to uppercase using the conventions
of the invariant culture, and then
performs a simple byte comparison
that is independent of language.
This is most appropriate when
comparing strings that are generated
programmatically or when comparing
case-insensitive resources such as
paths and filenames.
http://msdn.microsoft.com/en-us/library/system.stringcomparer.ordinalignorecase.aspx
InvariantCultureIgnoreCase
The StringComparer returned by the
InvariantCultureIgnoreCase property
compares strings in a linguistically
relevant manner that ignores case, but
it is not suitable for display in any
particular culture. Its major
application is to order strings in a
way that will be identical across
cultures.
http://msdn.microsoft.com/en-us/library/system.stringcomparer.invariantcultureignorecase.aspx
The invariant culture is the
CultureInfo object returned by the
InvariantCulture property.
The InvariantCultureIgnoreCase
property actually returns an instance
of an anonymous class derived from the
StringComparer class.
