Handling a whole sitemap containing 6000 links is that it could require too much time to complete generation and the server could timeout.

For XML you could use AJAX precaching with AJAX generation to generate sitemap by steps.

About the HTML sitemap, always avoid to include thousands of links, because also if the 500 error would not happen, too much data would hang the browser.

In your case i would suggest a strategy.

1- Generate a full XML sitemap, export as sitemap.xml then after positioning in public_html submit to GWT
2- Limit the articles to most recent and submit the link for sitemap dynamic generation to GWT

This allows you to have an old big static part of sitemap and another part managed in realtime quickly.

Cheers!

John