If you have to choose between those two, I would go for the Symantec branded product — it is basically (almost) the same anti-virus engine but with a more serious user interface and in my experience it is a little bit faster and less of a resource hog. That being said, I personally wouldn't advise either.
If you are taking it because it is free, I would highly recommend you take a look at Microsoft Security Essentials. It is free and a brilliant piece of software — much better than many pay-for solutions that I have tested.
On your second question: Malware is not common under *nix environments at the moment. Just don't download scripts and files from places you do not trust and run them as root! I have seen so many people get their Linux boxes attacked / taken over because they run random software under the assumption that it is free.
If you are the sort of person who downloads random stuff, then maybe you should — if you are careful... Don't!
However, the majority of AV for *nix is usually just to stop malware on Windows boxes, so I am not sure how helpful against scripts and hacks it would really be anyway.
