The basic antivirus and spyware protection is going to be essentially the same between all of the major programs, whether they are free or paid versions. Some will be a bit stronger than others, some will do a few points better or worse than their counterparts on the AV tests, some have a little higher or a little lower "detection rates" but the basic AV protection remains the same. They all do a fairly good job at blocking malware but no one AV program will block, catch or remove everything.
The difference between the free and paid versions is the free ones generally offer good antivirus and spyware protection but not much else in the way of extra features. Some others offer a few other benefits such as internet search safety scans or software update alerts. The paid programs usually offer other features that will "enhance your security" like Sandbox Browsers, super tough firewalls, spam filters, some claim to offer identity theft protection, child protection, there is a long list of what the paid programs offer.
A good free AV program will do fine unless you need or want all the extra bells and whistles. The extra features can slow down your computer though. The free and paid AV's all have their advantages and disadvantages. Some will try to install tool bars and change your search engine, some will install unwanted add- ons and extensions you may not want or need, some will update less often than you may like, there are a lot of differences between the programs.
Avast Free 2014 is a great program, it is always rated near the top in AV tests, both free and paid. It offers excellent protection, has continuous updates and alerts me when I need to update any of my major programs. It also has outstanding support either by email or in their users' forum if you ever have a problem or need help removing malware. It does not slow my computer down either. I like Avast but you will get many different opinions on which is best. I don't need or want any of the extra bells and whistles in paid programs. There are often legitimate freeware programs that will provide similar protection.