Endothelial Nitric Oxide Production (The most important for athletes) relies on the enzyme eNOS to convert Arginine amino acid to Nitric Oxide.
The enzyme eNOS is found in the body in limited amounts. Therefore, only so much nitric oxide can be generated at any one time because the enzyme is a limiting agent
Beet Juice 🥤
Contains nitrate, natural nitrate is converted to nitrite by bacteria on the tongue and then further in the stomach. As nitrite enters the bloodstream it is converted to nitric oxide.
Contains nitrate, natural nitrate is converted to nitrite by bacteria on the tongue and then further in the stomach. As nitrite enters the bloodstream it is converted to nitric oxide.
Pycnogenol® 
When Modex® is consumed, the Pycnogenol acts on the endothelial cells in the blood vessels to switch on the synthesis of eNOS (Nishioka et al, 2007).
As there is more eNOS then it is possible to produce more nitric oxide. Moreover, Pycnogenol activates eNOS- it makes the enzyme work faster so nitric oxide is generated more rapidly. This is very important for the active athlete.
Conclusion
Pycnogenol is effective in aiding nitric oxide production. When consumed it activates eNOS in your body and is a catalyst to rapid Nitric Oxide Production. It also has several other imperative functions for performance and recovery as it is a free radical scavenger which reduces oxidative stress caused by training and extends the life of NO in the body.
Beet juice does not activate eNOS and has a different pathway to Pycnogenol as an NO production aid. It relies on adding nitrates into your body that is converted to nitrite by bacteria on the tongue which may be converted in to Nitric Oxide when it enters the bloodstream. Possibly converting nitrite to Nitric Oxide is the only thing beet juice may do for performance.
NO PATHWAY COMPARISONS
Pycnogenol > eNOS > Nitric Oxide in blood
eNOS is found in the body in limited amounts, Therefore, only so much NO can be generated at any one time Pycnogenol acts on the endothelial cells (most important) in the blood vessels to switch on the synthesis of eNOS
More eNOS = More NO generated
Pycnogenol also nuetralization free radicals caused by exercise which ensures NO molecules remain stable, prolonging its availability (maintains greater levels)
Beet Juice > Nitrate > Nitrite > Nitric Oxide in tissues
Nitrate needs to convert to Nitrite by bacteria on the tongue and then in the stomach Nitrite then needs an acidic environment to be efficiently converted to NO. NO is mostly converted in the tissues of the heart and the liver and not efficiently in the blood or by endothelial cells where its is most needed for better blood flow.