I own a licensed copy of Imagaro Z and rarely use it. It's very awkward if you want to clean your vectors after you trace them and lacks certain basics one would expect from a program in this price range.
Flexi has, depending on how you count them, five different built-in autotracing applications ... which is appropriate for a program in its price range. It also features excellent path cleanup tools for after you autotrace.
The best bang for the buck though is from either CorelDRAW or Adobe Illustrator. Both have excellent autotracing built-in and adequate path cleanup tools.