I have used NOD32 for the last year, I have 2 years left on my subscription. I was having issues with them, so I did not reinstall it. Instead I am using MSE and it for sure uses less resources, and seems to do a great job. And as for free between MSE and AVG, AVG is good but I would rather have a Microsoft program with a Microsoft OS.
I've got NOD and MSE on my laptop... not noticed any impact on performance. However, I have noticed one would throw up warning messages about detecting trojans etc on one website and the other throwing up warnings on another website.
Not sure if Spybot does this silently (on my other pc) because it's never given me any warning messages like these two.
I have been using MSE for about a week now and I have seen no performance issues with it. I will probably go back to NOD32 in a month or so just because of my subscription with them, but I am really enjoying MSE.