PCB controlled impedance
If your routing a diff pair you need a ground plane underneath it that is continuous, as the capacitance between the traces and ground needs to be controlled to some degree. One of the best ways to do this is to have the top layer be signal and the second layer be ground. (There are more than one ways to have an impedance controlled trace, for simplicity I'll describe only one) 

The idea behind a microstrip is that the current flows on the signal, and the return current flows back on the ground plane below, this keeps impedance low and lets it function like a transmission line, an you can minimize bad things like reflections and attenuation that would reduce the signal that the trace carries. 

Don't use a power plane for the basis of a microstrip, this interferes with return current (needs to be through ground) and will result in undesirable characteristics of your transmission line. 

While it is possible to to build a microstrip transmission line with a power plane, this is a bad practice. Why? because if you do use the power plane you'll need stitching caps, and each cap will have on the order of 10nH of parasitic inductance, this will inhibit the return current. If your ok with increasing the inductance and lowering the rise time then go for it. 



Edit:

If your routing PCIE keep the diff pairs on the top layer with ground underneath, here is a guide:

PCIE Express Routing