Upon checking mass customer reviews and satisfaction rates from sites like Newegg.com, TigerDirect.com, and Amazon.com, it is quite clear that Western Digital external hard drives are more popular. They have also gotten overall better reviews, making WD's customer satisfaction rate slightly better than Iomega.
Western Digital vs. Iomega for Customer Satisfaction - Western Digital
Cool Features and Overall Styling
Western Digital offers some unique features on their hard drives. Notably, many of their external hard drives have capacity gauge lights, indicators, and great styling.
Iomega's external hard drives are colorful and vivid. They also offer some unique hard drives such as a hard drive that looks similar to a Mac Mini (stacks on top of Mac Minis), and a hard drive that resembles they styling of a Mac Pro. However, since the majority of people don't own Mac Minis and Mac Pros, this is not a huge benefit.
Western Digital vs. Iomega for Cool Features and Overall Styling - Western Digital