Whats Better?

Whats better, a R/Mo or a R/D? Plz explain.

neither is better than the other. Both can be used effectively.
Secondary profession does not mean much because you can change it during the game.
For PvP a R/Mo would be better because you can bring mending touch.

There is no better secondary profession. Each secondary has it's advantages depending on the build you use and if you play PvE or PvP. Some examples would be R/E (conjure with an elemental bowstring), R/Mo (mending touch), R/Rt (barrage+splinter weapon, nerfed in an update but still effective) etc.

It depends on what you want done.

R/Mo - Barrage+Judge's Insight; mending touch; hard rez; hex removal

R/D - Expertise + Scythe Mastery

Scythe Mastery.... nah
R/D = marksmanship + ebon dust aura

Absolutely. Pyre runs it very well Give him Volley and he will blind a nicely clumped group of Charr.