Thèrése is more widely known, probably because a lot of people find her more accessible than Teresa. Teresa wrote books about her mystical experiences (not out of vanity but because she was ordered to) that can be difficult to understand. Thèrése wrote letters and also wrote in her journal, which was published after her death; although she was also a mystic, her primary message was the simple one of the "Little Way" of spiritual childhood. She died at the age of 24, so remains perpetually young in people's imaginations. She also said things such as "I will spend my heaven doing good upon earth," a statement that has been taken to heart by a lot of Catholics who pray novenas to her. Some get superstitious (in my opinion) by, for example, expecting to see the image of a rose somewhere on the third day of the novena. I have a hard time imagining Teresa putting up with such nonsense (although she sang and danced with a tambourine, something I have a hard time imagining Thèrése doing). Thèrése is often called the Little Flower and is commonly shown holding an armful of roses with a cross, as in the picture below. If one of these saints is found on a rosary center, it's almost always going to be Thérèse.
Teresa had the guts to reform a religious order in the face of the Spanish Inquisition, which didn't look kindly on new ideas. She wrote her most famous book, Interior Castle, because her autobiography was being delayed so long by the Inquisition's investigations that her confessor decided to try a new tactic; Teresa wrote Interior Castle in third person when talking about her mystical experiences, which seemed to work, although you'd think the Inquisitors would get suspicious after reading "I know of someone..." so many times. She's commonly pictured with her quill and a book, with the Holy Spirit descending to guide her writing. The picture below is cropped from such an image. Thèrése didn't have to deal with the Inquisition, but faced a debilitating illness that she knew would lead to her early death, as well as a prolonged spiritual darkness. In my opinion, the best thing she did for the Catholic Church was to counter the heresy that says we can earn our way to heaven; she did this with her spirituality of depending on God for everything, as we would a loving father. (I have an essay on that elsewhere, written from a Tolkienian point of view.)
Both "Theresa's", though, would agree with the wording in the picture below - the text is usually called "St. Teresa's Bookmark," because it was found in her office book after her death.
Thanks to Shrinefairy and StLuke for permission to use images from their Etsy shops.