The difference between these two m.2 wireless cards is that the NGWMG.S is denoted as "Low Power" and the other is not. I think the S stands for "Smart", whereas the non-S version is Classic.
http://ark.intel.com/products/94150/Intel-Dual-Band-Wireless-AC-8265?wapkw=8265 Intel® Dual Band Wireless-AC 8265 Product Specifications
(Click on Ordering and Compliance)
Is there any downside to the "SMART" version, or is it all gravy? I have a desktop, so I don't care about power consumption. So, in my case could this low power feature actually be a limiting factor in some instances (i.e. weaker signal as a trade-off for lower power consumption, less compatability with legacy Bluetooth devices, or perhaps less bandwidth than Classic)? In summary, are there any trade-off's I need to make if I go with the S version.
You are correct, the difference between those parts is the low power feature of the 8265.NGWMG.S. The functionality and wireless features of both versions are equivalent; however, your computer may support one version, the other, or none, depending on how it was designed by the Computer Manufacturer.
If you are planning to upgrade the wireless adapter in your PC with the Intel® Dual Band Wireless-AC 8265, then we strongly advise to engage the http://www.intel.com/content/www/us/en/support/topics/OEMs.html Computer Manufacturer Support, and have it replaced with their authorized service provider if possible. Intel does not recommend end users install or upgrade the various Intel® Wireless Adapter.
Using an adapter that is not supported by the PC may lead to errors, system damage, or it may even be illegal. Please review the following documents for more information:
http://www.intel.com/content/www/us/en/support/network-and-i-o/wireless-networking/000005687.html Regulatory Information Regarding Hardware Installation or Upgrade
http://www.intel.com/content/www/us/en/support/network-and-i-o/wireless-networking/000005846.html Why Doesn't My Laptop Recognize My New Intel® Wireless Adapter?
The AC 8265 is a great wireless adapter, and as you mentioned, desktop systems normally use the regular versions (not the low power ones), however, the system manufacturer is the only one that can confirm if it is supported in your computer.