I highly recommend TrendMicro for server (and domain user) deployments.
I've never used bitdefender, but anyone's website metadata that starts with "award-winning antivirus software" makes me leery.
I think it's important to understand though, NO antivirus software is 100% bulletproof. There will always be variants and viruses that get past. The best investment is to train and lock down the users in the environment themselves. 99% of the virus/malware attacks we've dealt with over the last 5 years have been from user allowance.
For what it is worth, I've used AVG on both Server 2003 and 2012 with no issues and the cost is reasonable. So that sort of surrounds 2008, but no actual experience with 2008. I had some problems with Trendnet on Win7 Pro that was being used as a server, blocking or quarantining some legitimate program files, but that may have been other vendor's fault - don't know. It has just kind of made me leery.