As Hilary said above.
My main subject is birds & I tend to set metering to either `spot` or `centre weighted`, because in general I'm more interested in the bird than the scene.
(The other thing to consider is setting `focus points`. For birds I use `spot` focussing, either single or 9 points.)
It depends what you are shooting really & what your priority is, be it a wide landscape with an average exposure for the whole scene, or a small bird against a bright blue sky (or indeed the same bird in a tree against a darker background)
ISO will be affected depending on what the camera is set at & how wide (averaged over the whole scene for Matrix) or narrow (for Spot) you want the metering system to analyse.
ISO for a dark coloured bird will be different from a white bird, even if they are on the same perch, with spot metering, but if set to Matrix there probably wouldn't be much difference. (depending how far away they were from the camera/lens & how wide the scene is though.)
Not sure I've explained correctly.........or made it more confusing!
(it's easier to do than explain)
Next time you're out, shoot the same scene in all 3 modes then compare when you get home to see the difference.
Try a landscape type shot and also something smaller/nearer too.
btw,He who dies with the most toys, WINS!Nikon D800E & D700 bodies + Nikon 200-400mm F4 VR1, 50mm F1.4G, 16-35mm f/4G VR, 105 F2.8 VR macro, 70-300mm lenses. A couple of filters, Giotto tripod & ballhead. Lowepro Slingshot 302 AW