Reason? More things have to be taken into account if you have the battle pc, if it's an unknown opponent with unkown abilities, he could very well blitz you right out the gate before your brain has a chance to adapt- recognize what abilities/threats are gonna be - send muscles the data they need - respond. Spidersense alerts you to danger no matter what the source and in theory if it's possible for your body to remove itself from the danger (ie it's not moving too fast for you to avoid), the sense should do it on autopilot.
the SS is good for things which U cant see, but the battle PC is GREAT for making spot-on assessments to just about everything within yur particular POV at the present time.
if yur worried about things ya might miss , then the SS is perfect for ya, but if U want to be able to (just about) completely analyze the opponent(s) that yur actively dealing with...........its the B-PC hands down.