/blu pros: number of sources or attack speed means crap if /blu is effective. Can get a decent amount of dmg down to nothing and only have to recast buffs as they wear from duration.
Cons: Heavy dmg will still be heavy. Also if you are taking decent amounts of dmg you will be losing hate faster. And giving mob more tp to use tp moves on you.
/nin Pros: regardless of how hard the mob hits you will still take 0 dmg. Not much hate loss from shadows. Allows for casting knowing you wont get hit etc.
cons: If you can't keep shadows up your taking it too the face. So things that attack fast/accurately/multiple mobs doesn't work so well against.
And all that is somewhat relative to the amount/type of support you have.
While Dasva is absolutely right with his situation statements, in most of the situations where you would specifically want a RDM tank, you would want to be RDM/NIN, as RDM is normally used on mobs where Atonement spam or DD tanking isn’t viable. e.g.; TP feeding is a concern so /BLU isn’t advisable.
Attack speed is normally something you can control (to a point) with Slow II and Elegy, and it is rare they you need to be tanking multiple mobs, normally mobs should be slept or pulled so they don't link. Although there are a few super tanking situations where you do have to hold multiple mobs (DM).
RDM/BLU is normally reserved for supertanking, skilling up and messing around.
Do you mean capped stoneskin? Because you don't need 'full MND' set for stoneskin.
You need 92 MND as a naked RDM to cap stoneskin, which for most races is ~30 from gear. Rings (10), Neck (5), Back (4-5), Ears (1-4), legs (8-10) should be enough to do it. That leaves Head/body/waist/hands/feet for fastcast and haste.