The general consensus is that AVG is bloatware nowadays. I highly recommend Avast, although a lot of people like Avia (I haven't used the latter). Sounds like you can't go wrong with either.
I have experience with both Avast and Avira and both are excellent.
I use Avira on my computers. It is lighter on system resources, tends to have a slightly better detection rate (although both are very impressive). Avira's latest edition (Version 10) has a pretty impressive anti-malware (in addition to Antivirus) ability, too. The real downside is, you have to look at one popup, every day after updating, which is an ad for Avira's paid products. It tries to get you to register on installation, but just un-check the option, and you are good to go without registering.
Avast is really a suite of tools (Avira is just an Antivirus/Anti-malware). Avast requires registration and you get a key, which lasts for a year, via email. You simply renew your free key every year.
If you like a security suite, choose Avast. Also, I think Avast still works on Windows 95 (Avira does not).
If you are like me, and want to use individual components for various security issues, choose Avira. It now kills two birds (antivirus and anti-malware) with one stone, but there is no email protection, firewall, etc. Avira is also ideal for computers a little light on RAM.
As the Fezman said, you can't go wrong with either.
Just make sure you completely remove your old AV. With many that involves downloading a removal tool from the old AV's website. Norton, McAfee and in some cases AVG do not fully uninstall using Add/Remove Programs.
One last note: The new darling among the very geeky is G Data, which has kicked some serious ass in recent AV comparisons. It is reasonably priced (not free) and has a 3-PC price which is quite good. G Data offers a stand alone AV and a suite. I have yet to use it for a couple of reasons. #1 Avira kicks ass and is free. #2 G Data is still relatively new, and I like to stick with something that has been great for years. But if you are bold, and looking for the best, G Data is the new rage.
EDIT: Oh yes, AVG is bloatware and will slow down a computer with limited resources. It was a great product for years, but it can't touch Avira and Avast right now. I think it tries to install a toolbar as well, these days. That just pisses me off.