The Col: Motörhead and NP really rock! Although i have not been able to see NP live yet, but Motörhead is always great. If your clients enjoyed that, they very well might like Finntroll also. But of course MH&NP are easy listening compared to Finntroll (and Ensiferum who will be joining FT on the tour also).

But i'm not quite sure what kind of fanbase Finntroll has overseas and what kind of atmosphere would be on their show there. I would guess that they are not very well known on that side of the big pond? Maybe something can be judged by the venues they play at, for example DNA Lounge@San Francisco, El Corazon@Seattle, The Rebuplik@Calgary and Starlite room@Edmonton.

Anyways in your position i would atleast check out some reviews about their shows around there, or better yet go check it myself before taking clients to see them.