A 25 man guild I know did regular ragnaros on 10 man and then proceeded to kill shannox hc 25 before killing ragnaros on 25 normal, so one could assume that heroic shannox is indeed easier than normal ragnaros, at least from a 25 man perspective.
60 man ragnaros is harder than heroic shannox..
I think that Shannox 10 hc is harder than Ragnaros 10 normal but it think that it might be the opposite in 25 man.
Shannox 25hc is borderline free loot. We killed him in one hour after three pulls and we are a 1000 ranked guild.