I just ordered an Auber for my 18" WSM. Got the 6CFM fan. Should be getting it in by Friday. I hear they work well.
Below is per Auberins.com
Three different sized blowers are offered: 6.5, 10 and 20 CFM. A lower flow rate blower produces better control results at low temperature cooking (<250F). A higher flow rate blower is needed for a larger smoker and in cold weather for high temperature cooking. For Chargriller Akorn, only the 6.5 CFM blower will fit. For the 18.5" Weber Somokey Mountain, the 6.5CFM blower is recommended. For the 22.5" Weber Smokey Mountain, 10 CFM blower is recommended for low temperature and medium temperature cooking. The 20 CFM is good for high temperature control. However, a small percentage of customers (1-2%) reported that there was more than 10 degree fluctuation at 220F when using the 20 CFM blower.