Well... they're different brands. To someone not familiar with how pickups work they seem the same. EMG is a brand that mostly specializes in high output pickups. They have a series of active pickups that are their most popular ones.... the EMG-81 and 85's are the most common. Active pickups have an onboard preamp which requires a battery to run. This basically means that the pickup powers itself... allowing for great distortion and great crunch. EMG also makes passive pickups (no preamp). EMG-HZ pickups are the passive versions of their 81's and 85's. The DiMarzio and Seymour Duncan pickups are more traditional brands. They make passive pickups. There are differences in the types of magnets they use and whether they use individual pole pieces or a rail piece... how many times they're wound or over wound... so on so forth. It's like Fender to Gibson... They both do the same thing, there are just small differences that make them sound different. Most sites will offer sound clips of each of there pickups. If you're curious I suggest that you check that out to see how they're different. If you're not sure about the way a pickup will sound... just go to a guitar store and pick up a guitar with the specific pickups in it... and plug into an amp similar to the one that you have and try them out.