Some interesting comparisons here:
https://photopixreview.com/50mm-1-8d-vs-1-8g-vs-1-4d-vs-1-4g/
If weight is critical, the difference between the 1.4G and the old 1.8D (the lightest of all) is about the same as the difference between the F80 and F75. The 1.8G is only 30g heavier than the 1.8D, but significantly larger.
The G lenses have nicely curved aperture blades and render out of focus highlights smoothly. With the D lenses, unless you use them wide open, you see distinctive heptagonal highlights from the straight aperture blades, which are either cool or distracting depending on your point of view.
f/1.8 vs f/1.4 = 2/3 of a stop, which might be the difference between (e.g.) a shutter speed of 1/40 s and 1/60 s when light is limiting and you are using them wide open. And of course even shallower depth of field with the faster lens (though f/1.8 is already very shallow). The downside is a heavier and more expensive lens.
If you ever use Nikon digital, the current lower tier dSLRs (and Z series mirrorless with the adapter) only autofocus with the G lenses, since they are AF-S. However, if you ever use a classic manual focus Nikon or pre-F5 SLR, the D lenses are a better bet, as you'll need the aperture ring the G lenses lack.