Neither. Roman finger counting is radically different from how it's commonly done in the English speaking world today.

We have depictions from the 8th century that we have reason to believe are accurate representations of how the Romans of previous centuries would have counted. You can see one depiction here: http://penelope.uchicago.edu/Thayer/Images/Journals/CJ/47/2/Roman_Elementary_Mathematics/tesserae*.gif

You'll note that the number 1 is not indicated by either a thumb or the index finger, which is how we typically represent it. In fact, you'll see that the Roman method for finger counting was far more advanced than ours is. Through the use of both hands, one is able to represent numbers up to 9999. (http://penelope.uchicago.edu/Thayer/E/Journals/CJ/47/2/Roman_Elementary_Mathematics*.html)

If a Roman desired to express the number five, they'd raise their hand, and extend all their fingers but their middle one. If a Roman formed a V with their index and middle finger, with the thumb retracted, they'd be representing the number 22.

There's also an article in Classical Philology, Finger-Counting among the Romans in the Fourth Century, though its quotations from primary sources are in untranslated Latin.