The answer to your question would depend a lot on what you plan to create and make with your ShopBot. I purchased mine 8 years ago, purchased both TurboCad and Corel Draw soon after, and have never used either program to create a project.
If you purchase your ShopBot new, it will come with two programs called Partworks and Partworks 3D, which is written by a company called Vectric and identical to their products called V-Carve Pro and Cut3D. The only difference is the ShopBot branded programs only allow cut files to be created for a ShopBot, the Vectric products work with hundreds of CNC brands.
If you are going to invest any time learning a new program, I would suggest you start with Partworks/V-Carve Pro as it is a very easy program to learn and use, but extremely powerful with what it can do.
If you go to the Vectric website (links below) you can download a fully functional trial version of any of their programs. You can preview everything you make on the computer screen, but the limitation of the trial versions is you can't create cut files to actually make anything on your machine (with the exception of included samples).
After you download the trial versions, there are hours of video tutorials you can watch which take you through the steps of various operations.
Finally, the Vectric forum is a great place to learn and ask questions. Again, anything written about V-Carve Pro applies to ShopBot's Partworks.
Partworks 3D is a program designed to take a 3D model created in another program and create the toolpaths to machine it on your CNC.
The flagship product is called Aspire and you can purchase this directly from Vectric or through ShopBot. It has all the features of Partworks/V-Carve Pro plus adds a very powerful suite of 3D modeling tools for creating your own 3D models.
Here is the link for the trial programs:
http://www.vectric.com/WebSite/Vectric/products/download_products.htm
Here is the link for the tutorials:
http://www.vectric.com/WebSite/Vectric/support/support_vcw_tutorials.htm
I know there is a lot of info here. Ask if you have any other questions.
Good luck on your decision.
Tim