If you are looking for more definition to help determine desireable targets, better depth (stock coil vs stock coil), and are hunting on land.... get the Etrac. Its a known coin finder.
However if you are primarily in the water, and/or are fine with the level of target definition you could keep the AT Pro. Also if the ground is super duper hot a single frequency detector works a bit better.
In the end its really going to depend on what you are using it for, and where. The Etrac is a much more capable machine, but there are LOTS of good things found every day with the AT Pro. Its no slouch, especially for the money.
Hope that helps.