DPS usually cost more, esp in Summer. if you're talking about Japan +1 domestic destinations (which cost the same in miles) or Sapporo, that'd be a different matter.
(whoops i see i should be comparing J prices but I know nothing about the DPS market. my above analysis is for Y. Typical J fares to Japan for 2 people is 11000 HKD/head for packages w cheapest hotel. if anyone can chip in for DPS.)