Advice for choosing a replacement battery

Dell charges considerably more for a replacement laptop battery than do most on-line retailers specializing in batteries and offering after-market versions (with a one-year warranty, in most cases).  Anyone have direct experience or access to data that would argue for or against buying a cheaper battery?  And if I do go that route, are any of the on-line retailers (e.g., laptopbatteries.com, laptopbatteryexpress.com, PowerBiz, battery-biz, etc.) particularly reliable?  I have an Inspiron 600M, by the way.