I like both, but like
Tamsin more. That's because, while
Veronica is no doubt elegant and sultry, the "ron" syllable is a little off-putting to me. I'd be afraid to name a daughter
Veronica because I wouldn't want her to be called
Ronnie. And I believe that you can never be sure any intuitive nickname is 100% avoidable.
Then again,
Tamsin might lead to
Tammy ,,,,,,
I probably wouldn't actually use either, to be honest.