Re: We should use neither
> Its too low powered for most proper computing tasks
A Pi2 or 3 is far more powerful than most computers that ran Windows 98.
> and too "fat" for serious controller tasks where you don't usually need or want a full blown OS with all the apps and your program running in Python.
Develop on Pi2 or 3, deploy on Zero or Compute Module. You can't do that with Windows IoT. Linux can be cut down to much less than 'a full blown OS'*. You can develop in many other languages if you don't like Python.
In any case, a Zero is $5, how much less "fat" can it be ?
* There are many Linux distros that can (or could) boot up from a single 1.44MB diskette and run on a 80386 with 16MB of RAM.