Uma comparação da diferença entre os dois de Microsoft :
Q: How is this different from Windows Defender?
A: Windows Defender detects and removes known spyware only. It is not designed to protect against the full breadth of malicious software, and specifically does not prevent viruses, worms, Trojans, and other malicious software from infecting your machine. The new no-cost solution will be a comprehensive anti-malware solution.
Q: Is Microsoft Security Essentials designed to replace Windows Defender?
A: No but if you are running Microsoft Security Essentials, you do not need to run Windows Defender. Microsoft Security Essentials is designed to disable Windows Defender in order to manage the PC’s real-time protection, including anti-virus, rootkits, Trojans and spyware.