Short answer is it doesn't matter.
Long answer is it doesn't matter because no single workout is "best".
40 min at LT is almost a race effort, it will stress you a lot but you will need to recover from it for a long time (probably about a week on average). 20 min is more or less a standard Daniels tempo, you can run 2-3 of those a week and still recover. What is better, one big stress or a few smaller stresses? I think it depends on overall training plan, but smaller stresses feel safer and you can control your training stress better in smaller portions.
Some people believe that the best bang for your buck, especially while training for marathon, lies in longer sub-LT tempos that are proportionally longer (anything from MP to HMP range). One can speculate that you get benefits of LT training while running longer and at a more specific pace, making you more efficient at MP.