I have been running Sophos for well over a year now and I have never seen it being a resource hog, neither on managed PCs nor on standalone PCs. in fact the footprint is as small as anyhting.
Av0k, I have never worked with the managed solutions from McAfee or Symantec, only with the standalone versions, my last experience with both was a few years back and it wasn't exactly the best one. McAfee was a bad resource hog last time I used it and would sometimes for no reason use up all the CPU power. Symantec, if the comparative tests I read every now and then are anything to go by, is just not up to scratch when it comes to detection and protection. Again, anyone's mileage may vary and I'll gladly stand corrected.
Wong, the main reason for advising you to steer clear of Forefront is, that - unless they have fundamentally changed it - it is a completely sub-par enterprise solution. Managed installation is a lottery, it sometimes works, sometimes doesn't. The same goes for updates. The management solution did not allow for any proactivity, e.g. Sophos sends out mails immediately when it detects a problem on a managed PC - Forefront doesn't (didn't) do that. There is also no way to distribute Forefront over several locations that are connected via VPN and still have it centrally managed. With Sophos every major site of mine has its own update manager, but still I manage everything on one console. Forefront just doesn't have that facility. Sophos has the additional bonus of allowing for application control, blocking of USB devices and a wide range of other policies, though I am sure the other Enterprise solutions have similar facilities. We stopped using Forefront 1 1/2 years ago precisely because it was way too barebones and caused us more problems than it solved.