I tend to do better in a TT so I’ll count myself as a Time Trialist and yes I do prefer a 20 minute test. This is really only due to the fact it just feels like a complete workout and most of the TT’s I do are shorter as the TT scene here is not that strong so 20 minutes is my typical time for the race.
That said I’ve done both close to each other and pretty much got the same results.