Disable tags or categories: which one to choose?

Managing tags and categories at the same time is a lot of work and might by a little confusing for some users. I would like to enable just categorization or tagging on my service which will be more focused on website creation (i.e. the blog functionality will be secondary).

*First question : Which one to choose for my users? (For example Matt Cutts only uses categories: http://www.youtube.com/watch?v=A96yDPqa2rs
*Second question : Is there a plugin that could completely disable tags or categories (back-end and front-end)?
*Third question: Do you see any issue for my future service? In other words do you think that it’s a really bad idea?

Thank you for your feedbacks!