A completely broken sitemap (ie 90% of pages in sitemap are incorrect and generate a 404) or Not having one at all. And before you say just build a new one, I will be after a major site overhaul. I just want to know what is best for the time being
Get rid of the busted map... better to not have one at all (and don't forget to remove any reference to the sitemap from robots.txt)
I had a busted sitemap on a file due to the 50,000 page rule and didn't catch it for a week and my site ranking went to hell.... My NEW rule of thumb is to fix any warnings or errors immediately.
1) Get rid of the bad sitemap 2) Do your site overhaul 3) Resubmit your sitemap to Google, Yahoo, MSN
Delete the bad Sitemap Create a new one using gSiteCrawler Then continue to do your site overhaul. Then weekly create a new Sitemap and submit it via Google Webmaster Tools. This will give you a head start on reindexing the site.