I agree that
Tessa is too cutesy and I find it rather imature sounding. I prefer
Teresa with
Tessa as a nickname and I know a
Teresa who is called
Tessa by her siblings.
Although, I greatly dislike it when the formal name is chosen because of a nickname. My boyfriend's name was chosen because his parents wanted a certain nickname for him and he hated the nickname. In the end, his parents actually didn't like his formal name (what he only goes by) and chose it only for the nickname. I think the nickname should be decided after the birth, so the child can have some say-so.
Sorry for the soapbox there!