I have not used Avast for quite some time, so I cannot tell much about it (RAM usage, protection,...), but did Qihoo 360 Total Security (from now on: QTS360). It used to have both great (cloud) signatures (therefore, stable internet connection needed, for offline use, you might want to use Bitdefender and Avira ones too, but they come only twice a day AFAIK, and Bitdefender signatures happened to not install always on time). QTS360 HIPS served me well most of the time, too.
Also, GUI of QTS360 is easy to understand and nicely done. You can change to various themes if you like.
There are some minor tweaks I'd suggest to take from the stock version.
QTS360 autoallows chinese PUP/PUA which I really dislike.
Apart from that, QTS360 did very well trough the time I was using it. Malware Vault tests in the past months weren't that good unfortunately, but that might just be an exception, every Antivirus software can fail at some point.
QTS360 is relatively lightweight.
You might want to read my QTS360 review:
User Review - Qihoo Total Security 8.6.0.1158
Bottom line:
I'm not sure whether Avast autoupgrades to new programme versions, but Qihoo Total Security (Essentials) only offers upgrade to new versions if you trigger update manually (judging from last used v8.6.0.1158). This is annoying IMO and might put you to danger if something critcal is implemented in the new version.
Judging from
@silversurfer 's tests in the Malware Vault, I'd suggest Avast (find something useful here:
How to setup Avast Internet Security 2016 for Maximum Protection (Guide)).
But you might follow
@Spawn 's suggestion, which is highly recommended IMO.
I would recommend trial runs of Qihoo 360 Total Security Essentials and Avast Free Antivirus to see what performs better for your machine.
As with any security solution, Unchecky.com can prevent toolbars and other unwanted software from installing.
Perform a custom install with Avast as you will not need all Shields or other Components (ex: Cleanup, SecureLine, Mail and SafePrice extension). If you don't like avast, they offer a removal tool to clear Avast from the system.
EDIT: It is correct that Qihoo 360 upgrades Total Security much more often with new features / bug fixes than the Essentials Version.
For this reason, you might choose the non-Essential one and just not use the Cleanup tools.
Changelog:
Versionsverlauf | 360 Total Security
Versionsverlauf | 360 Total Security Essential