A guitar neck is one of the most difficult things to model.
Generally, learning the methods and processes is more important than learning the software. Meaning that regardless of the software, you'd generally go about it in a similar method, but using different commands, depending on the software.
I would watch as many tutorials as possible, regardless of the software used, to see the approaches being used. You may find that there are much better methods to use, and learn things you normally would never think of on your own.
There are a lot of tutorials on Youtube.
https://www.youtube.com/results?sear...+neck+modeling
I would expect Rhino to have a similar learning curve to Fusion 360, but it would be a quite different learning curve, as Rhino is a lot different from Fusion 360.
But I would say that these two would be the best choice.
Blender would have a steeper learning curve, as it has a variety of different types of modelling tools to use. But it's free.
Personally, I would not recommend using Aspire as a 3D modeling tool. It's designed to model 3D reliefs, and is not a CAD or 3D modeler. I know that quite a few people use it as an all purpose modeler, but I don't think it's the best choice.