I think you mean New Mutants 25
I don't think so, or at least cgc has the full appearance as 26.
First appearance 25, first full and first cover 26
Imagine if people actually opened the books to see for themselves what the "appearance" was. This one would be close and shut.
I've always gone by #25, but whatever you have the most copies of is fine with me.
I've never heard or seen anyone refer to 25 as being (more) significant.
This is the same situation as Wolverine, so if we keep it consistent, then issue 26 should be the book to get. I have already put up my copies for sale and so far a majority of the folks are hitting me up on issue 25.
That's what the wikia for him says is his first appearance. That's what most people go by I think. Wolverine's first is listed as 180 on his wikia....so...the market