Question Regarding "Coming into Play" or what ever Powers

[SomethingElse]

Let's say your opponent has an Ampharos PT in play, and a Miasma Valley, and you play a Dialga PT #5. Does Miasma Valley trigger before the power, thus making Ampharos cancel it? Does it trigger simoultaneously (sp?) and the owner of the stadium chooses? And if it was Dialga PT #5's stadium instead, would it change whether the power still works?

 

[PokePop]

The damage counters would be placed before you have a chance to active the Pokemon's Power.
The Power is shut off.