"My question: What's better for rendering? Cores/ Threads CPU? GPU?

It becomes murky to me when people use Superfly, because they can use their Video Card with tons of cores. I have a fairly decent card, lots of cores, but I don't see much difference in speed when rendering high quality, though you do get to keep using your computer as the CPU isn't buried."

For me when rendering my GPU is a lot faster at it than using CPU, but from my understanding this is different depending on graphic card and CPU etc. I don't have a fast computer, but an ok graphic card. But as Seachnasaigh said, you can run into problems with the GPU once the memory is filled. Meaning textures wont load and so forth. So I just switch to CPU when thats the case. Luckily they have started making graphic cards with a lot of memory, but as far as i know they are still pretty expensive, but guess in a few years they will be cheap.

Regarding Superfly and speed, its definitely not the fastest render out there, but there are some things you can do to speed it up.

These are my settings for Superfly which works well for Preview renders that are decent quality and support glass (Reflections and refraction)

If you need high quality simply increase the pixel samples to 40+ something, just leave everything else as it is. You can check "Caustics" if needed.

Second thing you should do, is always remove the default ground plain (Turn it invisible), did some test on this trying to render a single model and but removing this it decreased the render time by 70% or something. Why the ground plain is there by default is not really beneficial I think and have no clue why they would do that in the first place. But my guess is that most people just leave it there and build their scene inside it. But what SmithMicro have done, is pretty much just to add a huge object that need to be rendered and even though its grey it will take a lot of time to do especially when you start adding your own objects as well, also a good idea is to always render with an HDRI map in the background so you get something to reflect in metals, glass etc. The ground plain is in the way of that as well :). So only render things that need to be in the scene