Neither, actually! They serve two different purposes, normally, but you can powerfully combine them in this context.
Normally a sketch is intended to serve as the 2D representation of an outline you wish to extrude into 3d. It can (and should) be parametric and fully constrained. This means sketches will react to other changes you make in your design, and it's this capability that lends power to using a sketch for WHAT THEY WERE PROBABLY NOT EXPRESSLY DESIGNED FOR, namely as an anchoring entity for a joint.
Back to joints themselves: There are many kinds of joints you can make in Fusion. Normally it is straight-forward to precisely auto-locate points on any two objects and "joint" them using Fusion's powerful joint commands. When you issue the joint command and start to make your joint by mousing over the components to joint, you will see Fusion's recommendations for joint locations light up as the mouse moves. These are always corners, or centres of edges or faces of components. But it's often enough the case that none of these pre-programmed joint locations are exactly what you want, so you can use the offset field in the joint command to shift the final joint location from one of the default locations by some amount, along some axis. That works great most often, and the desired offset can be parametrically defined as well as statically.
Sometimes no default joint location will work elegantly or at all for you, even with offsets, which is why fusion supplies the "joint origin" command. This command is used less often, I suspect, but it is very powerful. It has two modes: between two faces (of the same component) or "freeform", which lets you put a joint location pretty much anywhere you want. The problem then becomes where, exactly, is "anywhere you want"?
To answer that, you can use a sketch to precisely locate any joint in 3d space, parametrically in relation to your model no less, by combining it with one of these rare but useful "joint origin" joints. (I think you could also use a regular joint at this point, so bear with me, I haven't tested) And this is where you can use a sketch for something it probably wasn't originally designed for, because sketches will take joints! In this case you won't be creating sketch to extrude something necessarily, (but you still could, of course), you will be creating a sketch that might forever remain 2d, never to be extruded, but it will still serve your purpose of locating that rare but handy "joint in space". (What ARE we smoking here? ;-} ). What've done is just create a sketch and draw a single centre-point circle, parametrically, fully constrained. That centre is now where my "joint in space" will be located.
I could go on, but let me summarize by saying that if you find yourself really stumped or frustrated that you can't figure out a good way to locate a standard joint using joint commands and standard offsets, then the ideas above can elegantly solve your problem.
