Its depend on what you building ...

If you building a blog or an informatics website then you want to go for custom development you can easy my interact with nibbleblog, WordPress, joomla etc CMS.

But if you are creating some specific functionality which is in your ming then you must go for custom development.

About WordPress
>Easy to install
>Open source -no cost
>user interface and admin good
>front end responce depend on your theme and plug ins
> little bit risky, people know source code easy to hack

About customs development
>time taking for development
>well secured
>apply any custom fictionality
>seo friendly as per development
>desired interaction and front end.