GPO Access and THOMAS for Legislative Research  is an excellent article at LLRX.com comparing and contrasting these two major government resources.
GPO Access and THOMAS are essential congressional research systems sponsored by the legislative branch of the U.S. government. Both are available online for free. GPO Access and THOMAS each take a different approach to legislative information and most researchers need to use both. The question is not “which is better” but rather “which is better for the task at hand?”