There isn't really a definitive answer to which is best because they've both got there pros and cons, and a lot will depend on the individual. But here's my observations:
No Lower Earnings Deduction
Access from age 55 at whatever it's value is at the time.
More investment choices.
No allowances are pensionable.
Lower contributions as a percentage of pensionable pay.
Investment returns will vary.
Your guaranteed at least the contributions going in.
Annual bonuses are guaranteed once added.
Higher contributions as a percentage of pensionable pay.
Pensionable allowances included.
You can pay into Bonusplan and get a little more off RM.
The Lower Earnings Deduction.
Low targetted annual increases.
No choice of investments, unless you also pay into Bonusplan and/or Flexiplan.
NRA of 65 with the ability to take it beforehand, but with a reduction.
*The pros and cons for RMPP members(sections A, B & C) building up a lump sum via the DBCBS, choosing to switch to the RMDCP are going to be different to the above.
*The plan is for CDC to replace both the DBCBS and the RMDCP.
The info rogersh suggests you read is all available here: https://www.myroyalmail.com/pensions
Also look in the RMPP plan guides for more info, available in the library sections of the RMPP website: https://www.royalmailpensionplan.co.uk/
And the RMDCP website: https://www.scottishwidows.co.uk/save/royalmaildcplan/