Not sure if I fully understand what you are comparing. The unit from Phototronix is likely brand new with a warranty and made specifically for your application.
Is the unit you have from the SB Chevy a marine unit with all the related items like matching coil & etc.., is it new or used, is there a warranty.
Kevin Smith, Richmond VA
1991 250/270 Weekender, 454 CI, 330HP, Bravo II