Its not a matter of taste, there are clear reasons to define traits on types or on objects.
Traits on types
We often want traits to work inside generated function (where we only have types), or on the eltype of an empty vector. Our target object we want to know the trait of is rarely a singleton - its the trait itself that is a singleton. So T.instance is not useful (its also touching internals). So we need to have a trait on the type.
Traits on objects
We cant define the trait on the type if we only know the trait at run time. So the type is useless. For example, we need this in GeoInterface.jl because some objects, like vector points or gdal objects pased in from C, can be 2d or 3d depending on runtime size, So is3d has to work on objects rather than types. Nearly always its a compile-time operation (e.g. for a Tuple or GeometryBasics.jl geometry), but having it work on the object means the interface still works when its not, its just slower.