"Last Modified" Googlebot question

[bc4u]

Hi all, 

I started off my XML sitemap by selecting "last Modified" "None" and added it to Google Webmaster tools...

I've now changed it to "Use This Date and Time" - As I just did a lot of updates on my site.

Will my third choice of "Use Server's Response" be better!? (Does that mean everytime I edit my site, it will be updated automatically so I don't have to keep re-submitting my sitemap?)

I don't understand the difference.

Does it matter how often GoogleBot will visit my site as to which option I select?

I havnen't been visited for several days and old pages still haven't been indexed, let alone my massive updates and changes!!

Thanks in advance.

bc4U

[admin]

Hello,

This field specifies the time the URL was last modified. This information allows crawlers to avoid recrawling documents that haven't changed.
You can let the generator set this field from your server's response headers or to specify your own date and time.

sitemap is not automaitcally updated.

[bc4u]

Thanks for your help