The widgets in tkinter are highly and easily configurable. You have almost complete control over how they look – border widths, fonts, images, colors, etc.
ttk widgets use styles to define how they look, so it takes a bit more work if you want a non-standard button. ttk widgets are also a little under-documented. Understanding the underlying theme and layout engines (layout within the widgets themselves, not pack, grid and place) is a challenge.
Generally speaking, the themed widgets will give you an application that looks more “native”, but at the expense of a loss of configurability.
My advice is to use ttk widgets if you want your GUI to look a little more modern, and the tkinter widgets if you need a bit more configurability. You can use them both in the same applications.