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One is ok, the other Google won't like.
If you are using SSI to include the text of the file within all your other files (the adsense containing file doesn't have <html> or <body> tags, it's just basically a text file that gets added by the server to each real page), then that would be the "good" way.
Thank you for clarifying this issue with us. The use of this server side include file will not be a problem as long as you do not direct traffic to the page [widget.com...] Your current implementation of this SSI will allow ads to be targeted to the content of your pages rather than the script page since the final HTML document contains the AdSense ad code.