The goal of assessment, regardless of test protocol, is to get a snapshot of your current level of fitness. And since your power levels are going to be based on your assessment outcome, we want an assessment that’s as close to accurate as possible. As a result, we recommend using whichever test format gives you the best representation of your current sustainable power.
Whether you use our 20-minute Maximal Lactate Steady State (MLSS) interval or a set of 8-minute Power at VO2max (pVO2max) efforts, it's best to choose a format that you will stick with and one that fairly represents your fitness at the time.
Check out the video below to find the best test for you :)
There's also a good explanation of the different ways to generate your FTP here at Alex's Cycle Blog. It's worth a read.
8 Minute FTP Test
20 Minute FTP Test