In general, when mocking up an interface in CogTool, you should use the widget that you believe your users will perceive the “interactor” to be. When CogTool doesn’t have the precise widget you think your users will be thinking about, you reason about which available widget is closest, as you did above. If you think your users will see a tab as a button, then use a button. If you think they will see a tab as a link, use a link.
Either way, in the frames you will probably want to use no rendering, with a background image for the widget itself, or a background image for the entire screen, because rendering a button or a link with CogTool skins will not get you the “tab” look you want.
For making predictions, this decision doesn’t change anything, because the choice of a widget type only impacts where Think operators are placed and buttons and links place Thinks the same way. you can read about this in the User Guide in Appendix C.