Audacity has some great functionality. In my opinion, Audition has a more "refined" interface.
Another benefit of Audition is the number of standard plugins that are included. Many of which are from Izotope.
Although I don't recall the Audacity functionality on this ability, I like the Audition "Mastering Rack". Here you can stack the various plugins and turn them on and off to hear how the gamut of the effects you are using affect the final sound. Once you are happy, you can "save" (although other programs say "render").
Over the last several months, I bought Izotope RX7 Standard and added the Ozone 8 Elements plugin. To my ears on my speakers, it seems that the Izotope programs sound a bit "cleaner".
While in the past I alternated between Audacity and Audition (depending on needs), these days I tend to use RX7 first and touch up with Audition (or vice versa depending on plugins needed) if necessary. The difference is that Audition has the "mastering rack" ability while with Izotope you have to render each manipulation before you move on to the next. (from what I've been reading, if you buy the full Izotope suite, you'd be able to approximate the mastering rack capability. However, I'm not studio recording - mixing instruments in separate tracks - all I need is post for a simple stereo live audience capture.