Last I tried Navicat it didn't offer much over pgAdmin. The UI was more streamlined but that wasn't reason enough to switch to it. What features in navicat are must have and not found in pgAdmin3?
One thing I like about pgAdmin3 is the ability to only execute the query you have highlighted and also the scratchpad. It's nice for exploratory querying. In Navicat, I like switching between various tabs where pgAdmin3 makes me switch between windows.
Both have some hilarious idiosyncrasies. When one locks up for no reason, I generally just alt-tab to the other.
Recently I was using pgAdmin3 and set my production database color to red. I happened to notice that the color didn't repaint back to green when I switched to my dev database. And Navicat seems to consistently hang whenever the network dies or burps mid-action, requiring a command-line kill.
AquaDataStudio was my choice at the time. Expensive, but worked on pretty much all the DBMS I had to use at the time (which included oracle, pg, teradata, mssql, mysql to name a few).