Which technique usually professional CI use for SEO?

[eluser]webbymonk[/eluser]
I am learning mod-rewrite for beautifying URL, but there is a lot of way,,
Especially CI already provided the segmented URI for easy query.

For example:
Old fashioned URL:
http://www.site.com/index.php?cat=123

CI style
http://www.site.com/cat/123

If I want to achieve http://www.site.com/cat/Event-and-Entertainment

Which technique is the best for this SEO?

1. using Mod-Rewrite to rewrite http://www.site.com/cat/123 to http://www.site.com/cat/Event-and-Entertainment

2. Do query directly not by ID but by category name?
ex: select * from category where category_name = 'Event & Entertainment'