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Question: Is the Wii version clearly superior in the gameplay than the Gamecube version? Or will I get the same enjoyment from either version?
Personally, I would rather play the Wii version. Basic gameplay didn't really change that much, but the pointer controls are very smooth and accurate. That might make the game easier, as you can quickly pop off round after round with precise targeting, but RE4 is all about empowerment anyway. If you really want, it supports GameCube controllers anyway - although I believe the way to get it to work is undocumented except through the internet. Other little touches (like the reload motion and use of Wii Remote speaker) are nice additions, and the extra play mode that wasn't included with the GameCube release is worth experiencing.
To me the Wii version is definitely the best available, and is easily worth the $10-20 asking price you often see it sell for.
However...
If you already have the game ready to go on GameCube, and if you don't mind the more traditional dual-analog controls, and if you don't care about the extra content available on the Wii version, then go ahead and play what you already own. RE4 on GameCube is still an awesome game and should be just as enjoyable. (Plus you'll never know what you are missing out on anyway.)