"Enriching" is subjective. You get out of life what you will. Many JET ALTs complain, as do many eikaiwa teachers. What do
you want and expect out of a job here?
Competitive? Sure, but that's no reason not to apply.
Grades in college are totally unimportant in either type of job. Sad, but true. Nobody here will even know what GPA means, so leave it off hyour applications. Minimum requirement is the completion of a degree,
any bachelor's degree. The college itself is also unimportant. Immigration just wants to see proof of the degree, whether from Harvard or Podunk U.
Look at the JET site and see what they want. Visit
www.bigdaikon.com and ask around the forum there (it's a JET haven), but beware of the many idiots, juveniles, and flamers there (many of whom will be your colleagues).
The job as a JET ALT and an eikaiwa teacher are pretty different. Hours, responsibilities, salary, class size, student motivation and mentality, etc. Their only similarity is in what you are expected to "teach" (oral communication), but even that can be different if you are the type of JET ALT that is used only as a human tape recorder. Experiences vary.