More pixels means smaller pixels or a bigger sensor. If your lens can resolve sufficient to put detail finer than two pixel widths on the P25 - which seems quite likely for a good lens, at least over the cantral part of the image - you gain resolution switching to the smaller pixels. I imagine this is the main motivation for the P45 over the other.
[IIRC there are either no or weak AA filters on both of these backs, so I said two pixels for the nyquist limit, as an initial guess - the limiting resolutions are roughly 55lp/mm and 73 lp/mm. A very good lens should just be able to make a difference at f11 or f16. I' expect you'd see a difference with a good 120mm f/4, for example.]
You are, perhaps, thinking that the noise is worse. Well it could be, but
a) you may be able to drop one ISO step, this should restore the noise
b) the noise is finer in the image, which helps
c) you can use selective noise reduction to obtain the same noise where it matters and better resolution everywhere else.
But please wait for the reviews before believing any of this, as the financial element is considerable.
EDIT: disable emoticons if you type b)!
2- A technical question: the P25 has 9micron pixels. the P45 has 6.8micron pixels!! Does this mean the P45 image quality is not as good as the P25'???