SilverStripe is much easier to use for the person who edits the website
- Pages are displayed in a tree control that replicates the website structure, making it much easier to locate the page they want to edit
- When they edit a page, the CMS displays the controls required for that template, instead of them having to know which custom fields to create. The controls are also the correct type (text, HTML, date, image, link etc) instead of text only.
When a web developer is selecting a CMS for their non-techie client, those two things alone are enough to choose SilverStripe over WordPress.
If you already have a site and you're happy with wordpress, I don't know why you'd switch. You didn't say why you are looking around for a new CMS. What are your reasons?