This may be a time-and-usage thing.
In the '40s and 50s, in Britain (London area) the term "crematory" was very common; these days, it is very rarely (possibly never) used here. Certainly, national news reports always use the second form so I don't think it is a North/South thing over here.

Second thoughts: [/b;ue]
My guess would be that "crematorium" sounds rather more up-market - the Latinate ending adds "class" to the whole dreary proceedings.