thank god for links....the easy answer is that studs are a little better.
some reasons:
you mention 'they are torqued to the same figure". actually the arp studs can be tighter due to the grade of steel used and the treatment. ideally you will use a stretch gauge which measures the actual tension, not a torque wrench which guesses the tension based on how much effort it takes to twist the fastener. they are rated to 190,000psi ish instead of around 120,000psi for a common head bolt.
bolts turn when you tighten them, with studs only the nut turns. this means the torque you are putting into it (and measuring) is spent only on turning the fastener not the whole bolt. much more accurate.
arp approach benefits:
arp cut the thread after heat treatment not before. this means when the thread is rolled in it is stronger at the points that take the most load. if you cut the thread then heat treat the resulting thread has even strength at the low and high stress points
arp design studs at the right length for the nut - the point where the nut/thread engages is the weakest on the fastener.
fine thread on top end = higher clamping at the same torque
fastener design - they have been very careful with design like wider diameter at the stressed area (threads), gentle radius to protect against stress raisers (were fatigue cracks start), machining for straightness last etc.
reasons some people say are a benefit but are not:
studs locate the head correctly. no way, jose. bolts are designed to clamp not hold in shear ie sideways movement. dowels are designed to locate a head properly
basically - I would expect a more reliable head to block seal under harsh conditions. I might only be 20% better but maybe that will make the difference
