I researched this a couple of years ago. Personally L-arginine is the way to go.
The use of Citrulline in an L-arginine formulation causes inherent problems, including depletion of L-arginine.
According to The Arginine Paradox (Folia Pharmacol. Japan Vol. 119 7-14:2002 Department of Pharmacology, Teikyo University School of Medicine) :
L-Arginine has attracted major interest because it has been identified as the natural substrate of nitric oxide synthase and is now recognized as a major player in the regulation of biological function.
The arginine paradox refers to the phenomenon that exogenous L-arginine causes NO-mediated biological effects despite the fact that nitric oxide synthases (NOS) are theoretically saturated with the substrate L-arginine.
There have been several explanations for this phenomenon, although none of them can explain the arginine paradox fully:
1. L-arginine-induced insulin, which has vasodilatory actions.
2. Neither extracellular nor intracellular concentration determines the NOS activity but rather the L-arginine amount transported across the plasma membrane may do so.
3. Endogenous NOS inhibitors reduce the enzyme sensitivity to L-arginine. These inhibitors include, NG, NG-dimethyl-L-arginine, L-citrulline, argininosuccinic acid and agmatine.
4. Intracellular L-citrulline, an NOS product, is a potent inhibitor of NOS so that the cells may need extra L-arginine to compete with L-citrulline inhibition .
Further evidence against the use of L-citrulline is the the 2005 Alternative Medicine Review reported that L-arginine, and not L-citrulline is the appropriate amino acid for oral supplementation:
“The amino acid L-arginine is the preferred substance for oral supplementation to enhance nitric oxide synthesis.”
“The mechanism by which L-arginine works is by providing the substrate for nitric oxide synthesis in vascular endothelial cells, which in turn creates cyclic GMP in the underlying vascular smooth muscle cells.”
“L-arginine overcomes ADMA’s nitric oxide inhibition.”
“Some supplement companies are marketing L-citrulline – a by-product of the arginine-to-nitric oxide pathway – as a substance to increase nitric oxide synthesis in vascular endothelial cells.”
“Citrulline does not directly convert to nitric oxide, but instead is recycled to L-arginine (an ATP-dependent process), which then converts to nitric oxide.”
The Nobel Prize in Physiology/Medicine 1998 was awarded to doctors Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad for their discoveries concerning “the nitric oxide as a signalling molecule in the cardiovascular system”.
As reported in the 2005 Alternative Medicine Review , “Ferid Murad, MD, PhD, Nobel-prize winner for his research on nitric oxide, has said the use of L-citrulline to increase nitric oxide is only marginally effective .”
The Alternative Medicine Review conclusion is …toss out the citrulline, and utilize L-arginine. “Change bad medicine to good medicine and ignore the hype.”