If you have the skills, I highly recommend building your own sitemap with large sites. I put it off thinking it would be too much work. It only took me about a half hour to write the script that creates a sitemap for over 10,000 pages (for www.zushop.com). How long does it take to run? On average about two tenths of 1 second! Compare that with how long I had to wait for http://www.xml-sitemaps.com/ to generate a site map. It is limited to only 500 pages and I gave up on it while waiting. Now I can just throw in a cron jog to regenerate it everyday - since my site is updated everyday.
nothing is impossible. it can hang your site if you run a cron job they way you are telling,slow process is good Regards Alex