Forms: Modal dialog or separate page in single page app?

In my application, I have pages that show lists of some objects, for example "People" or "Contracts". From these pages, the user may create a new object. Often these objects only have a handful of properties, maybe 4 or 5. My question is about the best approach for the forms used for creation of new objects.

Sometimes I have used a separate page, especially when the number of properties is greater than 5-6. Sometimes I have used a modal dialog. I have even seen some applications that insert an empty row into a table and you fill-in the values. (I will not be doing that!)

Any UX-preferences or suggestions for what approach to take? Also- is it important to be consistent between different types of objects? Or is a mix of approaches OK?

The technology of my application (Ember.js, single-page app) allows me to navigate to a different page instantly, so any timing difference between approaches is not a factor. In fact, the animation of the Bootstrap dialog takes longer than the page transition. The application is a back-office administrative system for insurance companies.