In your Google Sitemaps plugin, you have 'Rebuild sitemap if you change the content of your blog'. I am assuming that this means that whenever a post is added/modified, or when a comment is added, a new sitemap is created.
Why dont you have the option to only 'Rebuild sitemap if you change the content of your blog' when posts are added/modified? The blog admin doesn't mind the delay, but i would expect blog visitors would not like the added delay when they are posting new comments.
If I am making the wrong assumption, please let me know.