Besides the price difference, what separates the two options?
Is Windows Firewall, Defender and Microsoft Security Essential sufficient?
What kind of anti-virus or security solutions suits what user?
Security solutions are sometimes a personal choice based on need and budget. Some users are fine with free security. Others may prefer paid suites or paid individual products.
In general the paid products offer more features. While the detection may be using the same definitions, free products offer fewer scanning options than paid apps from the same company do.
One thing that free security applications usually do not offer is chat or phone support by the vendor other than via online forums. Something to watch out for with free products is the installation of components that the user may not want or need such as extra toolbars or trial products. These are usually an option checked by default. If such offerings are unwanted, be sure to uncheck them before installation.
To help users make a decision, there are companies such as AV-Test.org and AV-Comparatives that run tests every so often comparing the performance of paid products and free products.
Apart from actually having a limited budget of subscribing to or buying from off the shelf an AV/AM security solution would be the under the preference, know-how and geographical location of the user.
One view is that subscription based security solutions would be for those who can pay for it yearly leaving much of the computer based population with very little choice but to find a free security solution.
Regarding, "Is Windows Firewall, Defender and Microsoft Security Essential sufficient?", note that Microsoft Security Essentials (MSE) includes the anti-spyware engine of Windows Defender.