I got the 511 for it's compact size, approximately 13.5x8". It is perfect for my RV. We only use our rv 5-6 times a year and keep it dusted, however, there is always a smell and I sneeze like crazy. Since setting up the 511, my allergies have improved 10 fold. The smell has dissipated and the air smells clean. I no longer sneeze like a mad woman, so I can attest that it works. It is quiet too.