Most lenses have some amount of distortion, it just may not be measurable or significant.
Thom Hogan says of the f/1.4G:
Linear distortion is low (though slightly higher than the f/1.4G) and
barrel in nature. At under 0.5% it's not something I'd bother
correcting unless I had software that did automatic correction based
on EXIF data, in which case I'd just let that do its thing.
slrgear say this about the 50mm f/1.8
Distortion The 50mm ƒ/1.8 shows no distortion when mounted on the
sub-frame D200. On the full-frame D3x, there is a negligible (+0.1%)
amount of barrel distortion apparent in the corners.
Both lenses have 7 aperture blades, but that doesn't affect distortion. The f/1.8G blades are rounded, which will make the bokeh a bit more circular.
The more expensive f/1.8G has the following advantages over the f/1.8D
- sharper in the centre at wide apertures
- much sharper in the corners at wide apertures, most apparent with FX bodies
- AF-S focusing motor
- more expensive
- lack of an aperture ring
- bigger and heavier
- 58mm filter thread (unusual for a Nikon lens - f/1.8D uses more common 52mm filters
So in summary, I don't think the distortion is a deal breaker. You would buy the f/1.8G if you needed an AF-S motor built in, or if sharpness was a big issue.