I'd classify Nightwish as power metal.
Bear in mind that I'd do the same with Blind Guardian & Iced Earth.
Now note how the three bands are three points on the Clavicula Nox - that, of course, being a clever analogy with which I try to explain that they're not at all similar. Of course, certain aspects are akin between the bands, but that's just what makes them power metal, IMHO. The feeling, in the end, is what defines them. They are what your heart tells you they are.
Your heart can be dead wrong, of course, but look on the bright side - it could just be dead.
...
Right.
Goth? When judging if a band is goth or not, I'm biased towards defining goth in the limits of goth
rock and, in my knowledge, Nightwish doesn't have much to do with that. 'Kay, they've got
night in their title, but that hardly constitutes a firm goth basis.
The main difference between goth rock and goth metal is, in my experience (considering I'm more into the rock thing, but am a metalhead in my own right), the same as the difference between hard rock and heavy metal. They've both got heavy guitar riffs, bowl-shattering solos and angst-ridden vocals. How do we tell the difference? Well... we don't, really. Sometimes we do - when a band says they're rock or metal, it helps a lot to put things into context. When we consider the goth rock/metal schism, it's a little easier - if it's 80es, it's rock, if it's newer, it's metal. But that's simplifying, really. Technically, the label
goth rock applied to a myriad of bands (mostly) from the 80es, that evolved out of the 70es punk scene. Prog punk, in a way. Does it have Andrew Eldritch? Well, in that case it's not self-proclaimed goth, but let's be frank- what the hell is it, then?
But I digress.
I wouldn't say Nightwish is goth. They incorporate elements of goth, mayhap, but not enough for me to even put the label into their genre description.
As for the new Therion, that's prog, baby, with a touch of symphonic. Interestingly enough, it's prog
rock by the shape of its elements, but
metal by the way those elements are thrown at the listener.
It's Therion, damn it