-2EV means less exposure will be achieved below the metered value (although you can vary the metered value by spot metering). the rate of light entry does not change, but the overall amount of light entry decrease. the reduction of light entry may be accomplished by the camera's automatic shift to smaller aperture, shorter shutter duration or lower ISO, depending on your semi-automatic exposure modes.
R x E = I
rate does not change. exposure settings and light input, change.
light reducing filters decrease the rate of light entry, but overall amount of light entry will still follow the metered value or whatever EV that is applied. the decrease in rate of light entry is offset by increased aperture size, longer shutter duration or higher ISO.
R x E = I
rate of light entry change reciprocally with exposure settings to achieve the same metered value. any further change to I, such as further -2EV will bring along further changes in E.
the strength and feature of the light reducing filters determines your use.
CPL is good for bright well-lit environment but not overtly high contrasty ground and sky difference. it darkens the sky to contrast against brightly lit clouds and brightly lit buildings.
ND darkens all at the same time, through your viewfinder, but understanding that light accumulates on sensor over time, that does not matter. it is primarily used to prolong shutter duration in bright conditions while preserving the aperture and ISO or if the aperture or ISO is near the limits of the body/lens (smallest aperture/lowest ISO). alternatively supposedly that you have already set a short handshake-free shutter duration to the shortest the body can take, but it is still very bright and you will overexpose if you use large aperture, you can use ND filter to allow you still to use the larger aperture. i'm not an outdoor portrait photographer and not sure how frequent is it required to apply the latter, so maybe others can advise on that.
GND is primarily to control high contrasty situations.
if you just intend to underexpose everything, -EV should work (unless it is so bright that even -5EV still does not work, but i dun think you can get such a bright light). if you want to the details in the clouds to show, GND is the way to go, and less so by CPL.
while a GND can be very useful, another possibility is to shoot when the contrast is not so great. eg, early morning, late evening. strong side light, front lit building will give u well lit building w deep blue sky.
also, buildings stands like sticks on the horizon, so, while using a GND to darken the sky, one risk darkening the buildings as well. so, in this case, a polarizer is still useful as the polarising effect darkens the sky.
*my understanding is that from a basic meter reading, adding a -2 EV strength GND over the horizon will "under-expose" the sky by 2 stops, thus preventing it from getting "over-exposed" (the sky is ususally much brighter than the land), thereby preserving the details in the clouds (cus the clouds are finally correctly exposed).
*the polariser will darken blue sky, giving more contrast and distinction between the edge of the cloud and the sky, giving a more pronunced effect. details in the cloud may still be blown out, cus, both the land and the sky gets the same exposure even though they are of different brightness.
and if u ve both filters, i think its possible to combine them. ve read that many landscape photographers do this, though, personally, i ve little experience w filters.