After reading this thread on WinMatrix I installed BitDefender AntiVirus Free edition today. Here, I'm writing down my personal experience:
When I clicked on the big green download button on BitDefender's website, my Chrome browser asked me to save a 158KB exe file. I saved it on my desktop. The file was an online installer. Clicking on it started downloading a setup file of 8.25 MB on my desktop. When the downloading finished, a dialog box appeared with a message that, You are going to install the best antivirus solution in the world. The dialog box also inferred that Avast is the second best antivirus followed by AVG and Avira Internet Security. It gave me a comforting feeling that I was installing something good.
I clicked on the continue button and it asked me to uninstall my Avast Free Antivirus. I was thinking of using both of the antivirus' by disabling real-time scanning of Avast but it was not possible as I experienced. I had been using two antivirus on my PC already, Avast for real-time web and virus scanning and Malwarebytes Anti-Malware Free as on-demand file scanner. Clicking on the "continue" button launched the uninstaller of Avast. As we all know BitDefender's engine is better than that of Avast so I decided to uninstall Avast, which had been keeping me safe for the last 4 years. 
After a reboot I waited for BitDefender to popup the installer but nothing happened so I double clicked on BitDefender's setup file again. Now it asked me to uninstall Malwarebytes AntiMalware. I stopped for a while and then decided to sacrifice Malwarebytes also. I was getting under "B-Have" engine's protection then of what i was afraid of?
After a reboot the installer showed me a licence agreement and then asked me the location where I wanted to install the antivirus. The installation was quick. It reminded me again that I'm now using the best antivirus.
After clicking on the "Finish" button I noticed BitDefender's icon my system-tray area of taskbar. Clicking on it opened a widget like interface. There were only two options - first one was for turning the virus shield on or off and the second to disable the auto scan. At that point started realizing that why I loved Avast so much. Avast was not only protecting me but kept me informed that how it was protecting me, with its awesome graphs and logs. BitDefender was too simple for me. That might be useful for those who want to install an antivirus and forget about it but a data freak like me needs info, logs and graphs with sleek user interface.
Before uninstalling BitDefender Free edition I decided to reboot again and check it's memory consumption as BitDefender advertises it as the most light weight antivirus. I noticed two processes - gzsev.exe was using 50 MB of RAM and gziface.exe was using 4 MB. 
Avast runs only one process ie. AvastUI.exe which consumes only 3 to 7 MB of RAM. How BitDefender is the most light weight antivirus then?
I uninstalled BitDefender Free right away and re-installed Avast and Malwarebytes Anti-Malware.
TL;DR version: BitDefender Free Edition is too simple. Nerds won't like it.