Its a very funny question anybody in Internet marketing industry will come across.
To me if you ask the same question, I would say it doesn’t matter at all
Why
Search Engine crawlers are like a normal user, they cant see what is the code written at the server level to output a page. So what a crawler/regular user will be getting is a page coded in HTML. (I am using HTML as a generic term, leave behind xhtml , wml etc).
Again another argument is that having all the files with .htm or .html is advantageous in SEO. I cant comment on this strongly, but still as of now there seem no algorithm exist to give rating based on the file extension.
Stick to basics
Having .htm or .html extensions/files in a site is advantageous too. If they are plain html pages, they load faster as it doesn’t go through the parsers such as ASP parser, PHP parsers etc. When a file is requested by client, the web-server just need to respond with the file without processing the server sided code in it, because they don’t have any..
But nowadays the web-servers are quite capable to send out processed server sided scrips without much delay, so you cant really differentiate the response times of plain html and pages like ASP,PHP etc.
Another disadvantage of websites using server sided scripts is the over usage of query parameters in URL. Its very easy to use a single server sided page and show different page outputs depending on the query parameters. This should be avoided as far as possible.
One can make use of URL rewriting to hide such lengthy URL’s having some many special characters such as ‘&’, ‘#’ , ‘?’ and show a very friendly URL with an ending file extension .htm or .html !!