First of all I'm no expert. An "ex" is a has-been, and a "spert" is a drip under pressure. The two engines are close on horsepower, but I think the 502 makes more torque. The 454 had 11:1 compression which isn't pump gas friendly by today's standards. The 502 has 9.6:1 and works quite well with 92 unleaded.
It also has a mild roller hydraulic cam that can be swapped to make even more tire smoke. :
THEY BOTH FEATURE 4 BOLT MAINS,LS6 HAS 2 PC REAR MAIN SEAL,BOTH STEEL CRANKS,SOME FACTORY LS6'S COME WITH THE LS7 CROSS DRILLED CRANK,LS6 IS A MARK IV BLOCK,BOTH HAVE 7/16 RODS.AS STATED BY 12run THE 454 HAD 11.25 CR AND HAS A SOLID CAM SAME AS L/78-L/72.....1970 LS6 IS CLOSED CHAMBER WHILE 1971 IS CLOSED CHAMBERED PISTON AND OPEN CHAMBERED HEAD...COMP RATIO 10.25 IN 1971....502 HAD HYDRAULIC ROLLER AND LESS COMP RATIO........IN MY OPINION A 1970 LS6 PROPERLY SETUP,SLICKS AND GEARED IN EQUIVALENT CARS WOULD PREVAIL....WWW.HARDCOREPARTS.CA