Honest Answer, thanks!
I agree with Haje, the methods of CAD and art type modeling programs are fundamentally different. CAD is rooted in drafting whereas the freeform stuff is more based on clay modeling (from what I gather. I don’t use the freeform stuff.)
Freeform is the easiest 3D modeling software to learn for any application that is going to print or mill.
I run 3 seats of it and i can take someone who has never done computer modeling at all and they are fully productive in Freeform within two weeks.
Its probably the most expensive, however.
Modo is a good compromise between zBrush and a more professional design modeling app- although do prefer the way zBrush displays the model… i abhor zBrush’s UI and the worst one out there.
Fusion is also a very good modeling app though not for organic shapes… easy to learn and very powerful.
My advice is that different apps have widely differing approaches to how 3D space works and how operations are performed.
If you can- get evaluation copies of SEVERAL different apps- and try them out.
ONE of them will present 3D space in a way that YOU, individually, will find much more understandable and intuitive. than the others.
That is the one you will learn the fastest, and enjoy working with and the one you should invest in.
Don’t make the mistake of thinking free- or even lower cost is better. the best app will be the one that is facile given how YOUR brain interprets 3D objects on a flat screen,
I second the Rhino recommendation. I started in CAD with AutoCAD in 1982 and was a Beta site for more than 5 years. As a consultant I rapidly progressed to the point (Owner/VP) that I couldn’t afford myself as a draftsman. I always wanted 3D and believe me, Revit wasn’t it. I was a VP of a very large consulting firm in 2010 when I had a stroke, retired and started a new business. Nine patents and a lot of money later, I taught myself 3D using Rhino (at 69) and I love it. If you are in the Seattle/Bellevue area, I will share more for a pint or two. 
Well, there is a hidden gem that, to my surprise, I don’t see going mainstream.
I am talking about “Moment Of Inspiration” or MOI 3D (moi3d.com). This software was created by the guy who developed Rhino, Michael Gibson. So it feels like Rhino without the steep learning curve nor the tag price!
The interface is simple and powerfull, and what I like is that it could be used for both, engineering and artistic 3D modeling. The price is $295 and it includes 2 licences, one for Mac and other for Windows. It has a trial period, of 30 days as far as I remember.
Besides the software, you will found a very active and savy community of ppl that are very responsive. Michael, itself had answered all my questions when I don’t know how to create something.
As I said, it is a hidden gem, that is there ready to be discovered 