Hmmm, so you are traveling from Rome to Venice, via Pisa to check and claim your bags and spend 90 minutes total, to see the Leaning Tower, is that right? Up to you, but that will be both a lengthy day and a rushed visit. On a bright day, the tower will look lovely, just like in a good photo. On a dull day, it won't look as good. In your place I would either take a fast train from Rome to Venice and enjoy a more leisurely late afternoon and evening in Venice, or I would spend more time for a nicer and more relaxed visit in Pisa, and arrive later in Venice.