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.
Each test has its own pros & cons and I’ll elaborate on each of them briefly in order to help you choose a format which best suits you, both in terms of your current capabilities & capacity for suffering.
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.
The 20-minute test is the preferred format for newer riders primarily because new riders simply don’t have an established feel for riding well above FTP — which is what’s necessary during the short, pVO2max efforts necessary for the 8 Minute Test.
It’s also an ideal format to break into shorter, more manageable segments. By breaking the 20-minute effort into four 5-minute segments, riders can subtly tweak their effort every so often based on longer periods of muscular feedback, i.e. a rider can slightly reduce their pace every 5 minutes based on perceived exertion or how their body feels.
The downside is simply that it’s a relatively long effort and pacing is an issue for less experienced riders.
Additionally, many riders play it safe during the longer formats and end up finishing with too much gas left in the tank. Evidence of this is illustrated via a 2 or 3-minute surge in power at the end of an effort that would ideally be evenly-paced over the entire 20 minutes.
As a result, their FTP and relative power levels are likely to be underestimated a bit (or a lot) rendering all subsequent FTP-based workouts less productive than they would be otherwise.
Our preferred format for more experienced riders is the 8-minute protocol comprised of two 8-minute efforts. This format yields a greater amount of information by demonstrating power at VO2max via each 8-minute interval, sustainable power/FTP once the best 8-minute power is reduced by 10%, and you can even track improvements in your aerobic fitness when comparing the 2 efforts afterward, i.e. a substantial difference could indicate a limited ability to recover & insufficient aerobic capabilities.
Past the initial assessment, riders can also observe changes in FTP, and arguably as important, compare power at peak aerobic uptake/VO2max from assessment to assessment, information that can be of particular value to competitive riders and exercise science geeks.
A final consideration, whether an inexperienced or experienced rider, is the matter of keeping your assessment format specific to the type of rider you are. For instance, improvement in pVO2max usually isn't as important for an Ironman competitor or 40km time trialist as it is for a cyclocross racer or a criterium specialist.
This specificity is especially important when considering how assessment efforts are often themselves the day's workout, so why not work out in a manner that suits your training needs?
So our overall recommendation is to choose the assessment format that furnishes you with information specific to your personal goals and capabilities, a format which best mimics the demands of your events, and a format that ideally suits your strengths.
And try to recognize that assessment has its own learning curve and often takes a bit of practice. So while your early assessments might not yield the most accurate measure of your fitness, as you grow your fitness you'll also hone your ability to fairly assess it.