In the last block I have done both versions: 3x20 90÷/110% (30s) and 95%/110÷(30s). At 90% you basically feel lactate is “eaten” by your body very fast. Ar 95% it was hard workout - doable and my HR was lowering during unders, but it was a lot more taxating to aerobic system and muscles, when at 90% only legs were tired (I know there is relation between two but I am simplifying).
So 90% unders make the workout a lot more “user friendly” and the recovery is faster. I suppose that overall benefits are quite similar.