Definitely better. The earlier caliper was also used on 73-78 RD350 and 400, as well as the TZ250, 350 and TZ500 roadracers. It has two 48 mm pistons, where the 77 onwards XS caliper has a single 38 mm piston. With a 12-14 mm master cylinder, even a single disc setup is a great brake. The only reason for changing to the later caliper must have been cost. Also, the later one piece disc is a cheaper, simpler setup than the 73-76 two piece disc.