You provide two different toolchains for your Blackfin processors, VisualDSP++ and the Blackfin GNU Toolchain, and I'm not sure which to use for my upcoming project.
Which of the two toolchains is better?
Unfortunately there isn't a simple answer to that, otherwise we wouldn't provide both!
Which is better for you is up for you to decide, but I'll try to provide a few hints here.
The GNU toolchain's primary advantage is that it can be used to build Linux. Using Linux as your OS gives you access to thousands upon thousands of applications that have already been written for the platform. This can mean you can have a functioning application up and running much quicker than if you had to write all of it yourself on the bare metal. In general, GCC does control code and floating point fairly efficiently.
On the other hand, if you're going to be doing a lot of high performance signal processing then writing that code for bare metal using VisualDSP++ should provide you with better performance.
These are all, of course, gross generalisations and only you will be able to figure out which is best for your project.
For further reading on VisualDSP++ from a GNU/Linux perspective, please read the following: