Beta vs WHQL
I use WHQL drivers. The new 185 series have the nice ambient occlusion feature, but people have reported much lower 3DMark Vantage scores. Right now I use the 182.06 WHQL drivers. The 182.08 and 182.50 WHQL drivers perform slightly worse. I still feel my 3DMark Vantage score is unjustified, I have an E8400 @ 4.05 ghz and a single GTX 260 CORE 216 (EVGA 55nm model) overclocked to 684/1476/1190. My 3DMark Vantage score is only P13134.
In the past I saw no real difference with WHQL drivers and BETA drivers, but the 180 series is a different story. The 180.43 BETA drivers were some of the most memorable, performance increase was huge compared to the 178 series. I got another great increase with the 180.48 WHQL drivers, now there isn't much difference. Personally I read up what people say on the EVGA forums about the drivers and make a move. I like to be on the latest number (182 series for the WHQL drivers).