When it comes to computers, I prefer actual workstations, ones with Xeon processors and ECC RAM, while some may think that's overkill and/or that's just for the server space, well, a lot of the little memory issues that cause crashes on consumer products tend to not even cause a blip on this equipment (plus the error checking is offloaded to the CPU now, so the RAM hit on the checking isn't like what it used to be as well).
I also tend to use a real time kernel (low latency), but that's not going to be something that you are going to get with Windows. Haven't followed that much on Macs, but given their BSD roots, they have a better chance of better optimization then Windows (although I do have my concerns with them otherwise, but I digress).
If you stay the course with Windows rigs, I would suggest trying to remove any unnecessary process that is running while trying to do intensive work. That was a little trick that helped out with the Vista days. I don't know if they would allow turning off the eye candy with Win10, but I would do that as well.
As to the physical size of the case, I wouldn't go so far as to say that it has zero to do with power/capability as cases do in a way determine what hardware that you are able to put in there (as in physically fit and if it has poor cooling due to a poorly constructed case, that does dampen the power), but even if it's a big case, it could be filled with anemic components and therefor not as powerful compared to it's size.
Personally, I'm all for custom builds, if you are unable or unwilling or whatever the reason may be, to do it yourself, that I would suggest Signburst as well. Someone that you could tell your specific needs and they are more familiar with this industry. I wouldn't just suggest a techie friend due to the fact that the friend may steer you more to a gaming rig (or not a rig meant for signage as they may be more of a general techie and not as knowledgeable in the niche signage needs) compared to one for heavy graphics use. Although some people will say to just get a souped up gaming rig, gaming rigs, if powerful enough, will be able to get the work done to an extent (just depends on how demanding your usage is) that route may not be the most efficient route to go.
