Agree with the above 2 posts - i've also got maghari staff, abracadavrer, frost spike + scourgelords baton and niebelung. Tried it with a new frost offspec for trash in ICC 10 last night:
For bosses than need high / single target damage (eg rotface / saurfang) stuck to arcane with maghari staff for extra haste.
For trash, i used frost spec and niebelung. Was putting out (up to) 35K dps and had up to 6 valkyr out. Was good fun and amused my raid, but trash is trash, so is merely a gimmic.
Didnt get time to respec before green dragon (i'll try again next week) so dont know how that would go.
To answer OPs original question - Fire>Arcane for niebelung, but frost >> Fire. Unless its a big add based fight (Lady D, Lootship, Dreamweaver) i'd stick to maghari or equivalent for max haste.
Its a pity i've spent all my EPGP on what is really a gimmick staff, but actually i think this will be amazing for levelling in Cata.... just dont tell anyone so they dont nurf it!