Quick One - Surrey Wales today: M4 or M40?

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I'm off to Aberaeron, just south of Aberystwyth from Reigate, Surrey in about an hour, and was wondering which way the PH collective reckons I should go? I do this journey many many times every year, usually mixing my route up a bit to stave off boredom, but I have no idea what traffic will be like on New Year's Eve. Suggestions?

Thanks!

EDIT - It formatted out my little arrow between 'Surrey' and 'Wales' in the title

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http://www.highways.gov.uk/traffic/