RE: Nikon which one?
as heavy the 7400 is I doubt you'll notice the decreased size and weight of the 2x7 vs the 3x9. The one thing I can say about the 2x7 is that with it's lower mounting height, quicker mounting and shot placement should be possible with it vs the higher/larger 3x9. I put a 2x7x32 (Monarch) on my sons first gun last fall (mdl 7 youth .243) and it is a great scope and doesn't give up anything too most 3x9s. If you were asking about a short, light rifle I would say 2x7 all the way. But since it's a heavy fencepost like the 7400...
3x9!
RA