This is an interesting question I get asked all the time.
For the most part, there is no difference. That being said, style and sometimes size will determine the difference, especially for adults.
In my experience, most Bat Mitavah girls like pastel colors, sheer fabrics and more contemporary styles. I often have adult Bat Mitzvah women who want something a little more mature in color or more neutral colors like gold and white or silver and white so they are versatile with what they wear. The size for women and girls are virtually the same….18”x72” or 20”x80” which are standard sizes. Both girls and women will occasionally overlap wanting similar styles. There are some tallitot made for more petite girls when a 72” length is too long. I do suggest getting a Tallit for “life” not just for the Bat Mitzvah or matching a dress they will wear that day.
As for choosing a tallit for both men and Bar Mitzvah boys there is almost no difference in style but very often a man (because he is taller and broader most of the time) will buy a tallit that is 80” long. A taller Bar Mitzvah boy can also wear an 80” Tallit, but I find that I sell more 72” long for them.
Whatever your style or body type is, it all comes down to an individuals taste and how the Tallit looks on. It is important to buy from a vendor where you can either try it on before purchasing or be able to return it for a refund if it isn’t the right one for you.