The short answer is Grid North, and as far as I know, QGIS does not have the capability of distinguishing True North, regardless of preference settings.
As a flat representation of the spherical surface of the Earth, displaying features in the computer screen requires a transformation, the same as displaying features on paper. Certain projections (World Mercator, Plate Carrée (often called "unprojected")) will have meridians that run completely vertically, and therefore grid North will be exactly the same as true North, at the expense of distortions in size (for Mercator), shape, or both (for Plate Carrée).
For other projections, true North will not consistently be "up" everywhere on the map. For example, for a UTM zone, true North will align with grid North at the central meridian, but as you get toward the eastern or western edge of the zone, grid lines will still be oriented vertically while the true North will "lean" toward the central meridian. An unsourced eHow page explains how to calculate UTM grid convergence, and shows by example that the difference between grid North and true North for New York City is 0.65°.
To get back to your question, note that while we often use "WGS84" to refer to unprojected lat-long, it is a datum that can be used for a variety of projections, not just Plate Carrée. It is used, for example as the basis for Web Mercator, the projection that underlies Google Maps, OpenStreetMap, etc. Other datums, such as NAD83, can also be used for Plate Carrée projections. For example, the US Census Bureau releases their boundary files in lat-long (Plate Carrée) NAD83, not WGS84. To compare the two examples you gave, WGS84 (meaning unprojected lat-long or Plate Carrée) and WGS84 UTM, both use grid North, but for Plate Carrée, there is no difference between grid North and true North.