They are both good. I don't know which one to use, I like Navicat's simple layout but I think that Sqlyog may have more features. I also tried MySQL Explorer - http://www.mysqltools.com/mysqlexp.htm which also includes a nice diagram feature. Anyway they all have 30 day trials so I'll play around with them for a few weeks.
If anyone has any feedback on these products I would be most interested.