Because L-citrulline is more readily absorbed than L-arginine, which is rapidly broken down in the liver before it reaches circulation, L-citrulline is a far more efficient way to increase blood arginine levels. One study in heart patients found that 3g of citrulline (as citrulline malate) was equivalent to a 6g dose of arginine, suggesting that citrulline may have twice the potency of arginine itself when it comes in increasing arginine levels in the blood stream.
Doses of citrulline in the 4-8 g range seem to be optimal for boosting pre-workout NO levels, and can be taken as free-form L-citrulline as well as citrulline malate. For the purposes of increasing blood arginine levels, these are probably interchangeable...