A big difference is output. Have you checked out Sanjay Joshi's lighting tests? His test are fairly reputable, his site is:
http://www.reeflightinginfo.arvixe.com. Just use the search performance data for 400W, or read his articles.
The most light output comes from the 400W HQI ballasts, which are actually a High pressure sodium 430W ballast. You can buy them from PFO or your local hydrophonics store.