You will still be protected just fine with the free version. There's no difference in the quality of the actual scanning and detection-treatment of viruses and trojans, etc. The only real drawbacks are the nag screen to buy during updates and the lack of a few functions like live email scanning. If you want the extra features in the Premium, that's a personal choice. But almost all of those extras that are "bundled" into the one program can be found in other freeware programs.
For example, here's what I have on my system and others I can recommend.
Antivir Free
Sygate Firewall (discontinued, but still an excellent firewall. I score Stealth at the online firewall tests.)
PopPeeper (to pre-scan and delete junk mail before I download my mail to my computer, which we've already discussed.)
SUPERAntispyware, Spybot, Adaware (to manually scan for any nasties that might have gotten through. Although with Sygate Firewall, I don't get much. Usually just a few tracking cookies get through now and then, but not often.
My default browser is Firefox, with CS Lite (Cookie Shredder Lite).
AntiVir, Sygate, and CS Lite with FF are the only live protection I run on my computer. I keep things clean with PopPeeper and SUPERAntispyware.
In the event you suspect something got through there's always a fully functional 30 day trial of Trojan Remover to dig deeper, or other clean up utilities out there to use. AntiVir has some free cleanup tools here too:
http://www.free-av.com/en/download/index.html
Like I said, it's a personal choice. For myself, I don't want a lot of stuff running in the background. So I usually stay away from the "suites" that do everything at once. So far...so good.