Both areas are good neighborhoods. However, I consider the area south of Lambert to be a better buy as long as you are East of First Avenue. The area between Leffingwell and Lamber has the railroad tracks running through. This may affect the future value of your home, depending on what happens with the railroad tracks. To say the least, you have the noise from the train, which passes by once a day. I've lived in Whittier for the past 21 years, and I would have to say south of Lamber would definitely be my choice.
If you have any other questions regarding Whittier, feel free to contact me any time.