Lyndon:
Many companies who produce software will release "public betas" before finalizing a new release of the program.
Public betas ('beta' software means test software) serve two purposes: First, they enable large scale testing of the program by a base of users who are both knowledgable about the previous version, and enthusiastic about the new release - this allows the developer to find defects or unwanted surprises before the final release is made.
Second, the public beta trail gives the enthusiastic users (those who download and use the beta software) a way of commenting and criticizing the proposed new program before it is finalized. It is much easier to make changes to a software program while it is still in the 'beta' stage (e.g. not finalized) than it is after the release has been finalized, translated into 12 different languages, etc.
In the case of Garmin's MapSource program, if you are not both entirely familiar with the program already, and willing to put up with the headaches that can sometimes come from testing beta software, stick with version 5.4, which is the most recent "final" release. Any of the 6.xx versions are test (developmental) releases.
For what it's worth - MapSource 6.xx introduces a number of new features, but if you are primarily using the program to move map segments to and from your GPSR, you won't really notice much difference.
PanEuropean