For me, the pyramid sets have been the most time-efficient training-technique to get faster...

To answer the OP's question, the 6x6 is completely different than the 2x20. You'd do the 6x6 at 2-5% above LT (fastest pace you can hold for 6-minutes). This won't actually raise your LT-HR, but will increase muscular-efficiency so that you can generate more power at LT; you're using your oxygen more efficiently. Since it's above LT, you'll also generate more lactic acid and this will work on buffering.

While the 2x20 is a tempo workout right at or just below LT. This will improve LT and aerobic capacity, but not work the muscles like in the 6x6. You want to incorporate both workouts in your training regimen.