In the Attitudinal half, you receive information about what people say they would do or what they say they want. A lot of surveys and interviews can be unreliable. User’s memories are fallible and can miss details that are important to design. People also say things that they think you want to hear so as to not be impolite or against the grain. That said, some simple survey questions can be revelatory when related to user’s motivation, as in “why are you visiting the site today?” This combined with another simple question, “were you able to accomplish your task?” can give great insight.
The behavioral half gives you the advantage of seeing what users actually do not just what they say they’ll do. Web analytics can give huge amounts of quantitative data that you can’t find anywhere else, but the caveat with all that data is that it doesn’t tell you why like surveys will and it doesn’t tell you how. Deciphering how users interact with a product detail page or how they navigate from page to page isn’t easy with web analytics. There are pathing reports but they don’t help much. This is where usability testing comes in. It allows you to watch users and learn how they attempt to perform tasks. This gives you insights that aren’t accessible from any other source.
Relying solely on one source would be a mistake. Combining any two is worth more than the sum of their parts.