I've always thought it displays "Personalize this Design" only if the design includes any template fields in which case clicking the button opened the template panel on the product page, and "Edit Design" if there are no templates, in which case clicking the button opened the Design Tool.
I just clicked some random products from the MP that had text that could have been template text or graphic text and oh my, what a freaking mess, can't tell if something is/was supposed to be a template field or not. Ugh!!! So I went to my own store via an anon not-signed-in browser where I know which products have template fields and which don't. And the above proved true. Products where I have no template fields on them show an "Edit Design" button. Products I have included template fields on show a "Personalize this Design" button.
With all the madness happening with the changes to the customer interface, YMMV.
When you're designing the product, you need to set each text field as a template individually. You select the field and then chooss "Make this a template object" at the very bottom of the right hand panel. You can give each field a name ("product page label") for the customer to see when they're customizing the field. (Some versions of the new design tool don't show this field name, so I wouldn't worry too much about what you call each field.)
The bigger issue is that you can only set a field to be a template when you create the product, you can't edit that later unless you replace the design.
