I've never known a
Theresa whose nn was either
Tessa or
Tess. They all seem to either go by the complete form or use
Terry as a nn.
And I've never known a
Tess. Only in the
Thomas Hardy novel,
Tess of the D'Urbervilles! And her sister's name was something like
Liza Lu, I think. Anyway,
Tessa people I know are sometimes called
Tess but usually they just stay with what they've got.
Theresa seems very Catholic; certainly where I live that would be anyone's expectation. And
Tessa is denomination-neutral - I've never known a Catholic one. So that's the dividing line here.
According to comments here, people in the States seem to prefer
Tess to
Tessa. Is that right, or is it pretty much a namenerd option?