Depends on what you need. I have the Raven, and it rounded and great for making 6" wide holes quickly. The Piranha has a "V" shape with a wider top, so it can make a larger hole and move more dirt. I have been very impressed with my Raven so far.
I like my Raven. I can cut straight down on all four sides and make a semi-round "plug" that is easy to remove. When I'm done, dump the dirt in and replace the clean plug.
I was gonna get the Raven, but i called Predator Tools and George's son said that he preferred the piranha, so i went with it. It digs good. I got the footpads with it. They added a little bit of weight, but i like it.
I bought one of the first Piranhas that they ever made a few years back - and love it. It makes an awesome plug and a nice neat hole for those deep recoveries!