AVG used to be very light & fast, but it's got more bloated in the last couple of years. Constant upgrades got annoying too. Security Essentials is less intrusive. We're now running Avast at work, it had the least impact on performance, I tested several over a few months. It's highly configurable too, where AVG locks a lot away from you. Avast also works seamlessly with Windows Defender (spyware type prog) so both can run OK.
Some AVs can actually work together, they'll state which ones can co-operate with it. But whether the performance hit is worth the extra bit of security, who knows.
I've used avast for years, must have installed it on over 100 machines over the years, some paid for (commercial use) but mostly the free version, never had a problem yet.
Never tried AVG as i've never needed to.
I can put up with a little popup every now and then, after all they do give it you for free!
I used to put AVG free on customers machines as standard, but recently it seems to have grown bloated and tends to slow down older machines, and even new ones with a lower spec.
So I generally use Avast or Avira free instead now, as these have a lighter footprint.
I still use AVG on one of our home machines, but as someone already suggested it's best to turn most of the extras off.