The feel of the spaces and the zoning are somewhat different as each town has its own face and character. You really need to see it for yourself, drive around a bit, and you'd be able to see some of the difference yourself.
Both places are considered an excellent choice of community for families and some choose one over the other based on proximity to work and the time it takes to get there, considering specific routes and traffic patterns. Many buyers, however, who are likely to consider Lexington would also consider Wellesley and choose one or the other based on an individual property (its size, price, privacy, etc.) rather than town.