They both use a hex modifier, and a very similar, if not the same, encryption. However, I have also heard that the Action Replay allows for save game states. Essentially, to me, its which one supports raw hexadecimal.
Regardless, the only cheat device I've ever owned is the Pelican Monster Brain, which worked solely with the first six Pokemon games, however being that you could specify numerous mods without hex codes, and since you could make about 50 save file backups, I did find it a very useful utility.
Somewhat inactive Pokezam webmaster - you'd best talk to Legend or another moderator for advice.
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