Both are effective, but I would prefer the alpha-ketoglutarate form as arginine is bonded to a Krebs Cycle Intermediate in that case. This will make arginine more potent and would deliver broader overall benefits. Here is why:
The alpha-ketoglutarate will act as an effective transportation medium for the arginine. This will imply that less arginine AKG will deliver better results. Furthermore, AKG strengthens the endogenous production of glutamine and glutamic acid, both important compounds for immune modulation. AKG binds to ammonia in muscle cells, in the kidney, and in the brain, to form glutamic acid. This improves cellular glutamic acid levels and leads to excretion of ammonia. Also, AKG, as an important Krebs-Cycle Intermediate, improves VO2 Max and energy production in muscle cells, making it an effective ergogenic.