The answer is neither (no matter how you look at it). This is something that non-Chinese speakers are often very confused about. Let me try to explain this:

simplified & traditional refers to the script. The situation is analogous to the Latin and Cyrillic scripts, both of which can be used for writing Serbian.

Mandarin and Cantonese are the actual dialects (or languages, depending on your definition). The difference can be likened to that between British and American English but greater.

Some generalizations can nevertheless be made, though, as with all generalizations, they are not always true:

– If something is written in simplified, it is most probably Mandarin, and most probably Mandarin as spoken in mainland China

– If something is written in traditional, it could be either Mandarin or Cantonese, though Mandarin is more likely, since standard written Chinese is actually Mandarin in written form

– Cantonese is considered a colloquial language and is not supposed to be written down. When it is written down, it is most probably written in simplified in mainland China, traditional in Hong Kong or Macau; elsewhere it would depend

– Mandarin when written is usually written in simplified in mainland China and traditional in Taiwan; elsewhere it would depend. There is also a small chance it is written in something else (e.g., in phonetic transcription form, such as in some ads)

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Note added at 51 mins (2011-05-10 18:07:35 GMT)
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IMHO it is much more useful to talk about regional differences. Normally we talk about mainland China (zh-CN), Taiwan (zh-TW), and Hong Kong (zh-HK).

zh-CN is usually understood to be Mandarin written in simplified characters. The vocabulary and to some extend grammar is typical of the mainland China region.

zh-TW is usually understood to be Mandarin written in traditional characters. The vocabulary and to some extend grammar is typical of the mainland Taiwan region.

zh-HK is usually understood to be in traditional characters and the language is usually a school-taught form of stylized Mandarin. The vocabular and grammar is typical of the Hong Kong region; in addition, some of the grammar can be considered to have Cantonese influences.

Unless we are talking about transcripts of actual speech (video transcripts, scripts for spoken TV and radio ads, etc.) or an intention to, for whatever reason, write down colloquial speech, it is usually quite meaningless to speak of translating to/from Cantonese/Mandarin (especially Cantonese).

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Note added at 54 mins (2011-05-10 18:10:25 GMT)
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Sorry for the many typos in the note.