I think you're alittle off on your assumptions.
First, it's unknown at this time what the additional Mag.Att bonus is on the set, the Robe alone has +5, I am assuming you missread the Item description and made the association that the set bonus was +5, it's not. Second, I highly doubt a RDM could get a dalmatica quicker than a Morrigan's Robe. While a morrigan's robe takes 3 Pieces, one being a bullshit drop rate, you can still in theory attempt to get it more frequently. The main prohibiting factor is the Cost of the ingots in todays ecconomy, but coincidentally, they cost almost as much as what I see cursed Dalms go for in the downs. Back to the point, Nid pops once a week at best, Aspid pops once a week as well. they each have a 100% chance of dropping 1 Abj, and what appears to be a 50% chance at dropping a second. Nid shares that drop with N.Legs, M.Body, and E.Body; Aspid shares it with M.Feet, D.Feet, and W.Body. So you get a 25% chance at aquiring a A.Body on Nid a week, and a 25% chance on Aspid. As well, the 50% x 25% chance at getting it to drop on the 'second' abj. so between the two, you have a total chance at roughly 75% chance at it dropping over all. However... when you factor in competition, and Botting, if you're a legitimate LS, your chances drop exponentially. On top of that, you need to be able to kill the mobs in question.
SO where 1 pretty much is exclusively the droprate on 3 rats you have 100% chance at claiming in Salvage, the other is a 3-6 day window for one mob to spawn and a manditory 3 day wait in between, while competeing with 120+ people that lie, cheat, bot, and hack.
Unless you are in said linkshells. I think your chances of getting a Morrigan's Robe is much higher.
Did I mention you don't Schedule Nidhogg like you can slavage? Good luck wakign up at 3:30am to try to claim on most occasions....