I use both products: WebTrends ($10,000s) with my Fortune 500 company and Google Analytics (free) for my personal web stuff. Here are some key differences:
Cost: Google Analytics is free and WebTrends' cost is dependent upon the number of page views your site receives. It could potential cost in the $100,000s.
Capabilities: WebTrends has way, way, way more capabilities than Google Analytics. WebTrends is much more robust and customizable. Google is not very customizable at all.
Data security: Google collects the data and hosts it for you. WebTrends can do this as well but it can also be hosted on your own servers.
Time to implement: Google is very easy to implement and much, much less time consuming than WebTrends. (However, see capability)
Summary: If all you need to know about your site is how many visitors go to different pages in your site, then Google is all you need and the price is right. If you need to do things like segment your visitors in different ways, perform analysis across domains that you own, or perform other advanced analysis, then you need to go with a solution such as WebTrends. Is WebTrends the best one out there? It is one of the least expensive solutions out there. If your site gets a lot of visits, then I highly recommend Visual Sciences solution. It is by far the most efficient product out there and is incredible. It is a pretty expensive solution from what I hear though. If you don't have that much to spend, I would go with a WebSideStory before I chose WebTrends.
Hope that helps.