Thunderbirds ARE Athlons.
"Thunderbird" was the codename for the current generation Athlon processor (0.18 micron, on-die cache). They're still referred to as "Thunderbird" by many people to distinguish them from their older off-die cache versions.
So, the real question should read:
Classic Athlon or Thunderbird?
The answer to that, of course, is Thunderbird.
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