AOL is better, but it has less features and not configuration friendly. Symantec uses less resources at the extremes, but sometimes acts strange by not giving you all the virus and file information on detected viruses.
With Symantec, you need to set the Heuristics to the highest level, it doesn't slow it down to much but helps pick up more viruses.
With AVS, Do Not activate the high protection option as that can kill the system when it scans, but in Medium, make sure you set it to SCAN ALL FILES.
Do you have Symantec for free? If you have to pay for it, you might as well get the real Kaspersky. It is better, more configurable then AVS as it is the full blown version and it may be more stable as well.