20/20 (or 6/6 in metres) is awful. I didn't know that until optometrists started moving from cards at a distance (where I could choose which line I wanted to be able to read, thank you very much) to the retinal projection thingies most of them use now. The difference is, as you noted, about -0.25 for myopia in the -3 to -4 dioptre correction range, and it's huge. I'll assume that it's a better set-up than the old way for astigmatism (which was never a problem for me), but settling for "normal" vision when there's an option... I don't think so.
If you know a bit about optics it's not hard to derive. The difference between correction at 6m vs infinity is 1/6 - 1/inf = 0.166.. dioptres.
On top of this, one thing that surprised me was the difference in correction between red, green and blue light. The refractive index of the eye varies according to light frequency such that blue is focused approximately 2 dioptres more than red. If your myopic eyes are corrected for red at infinity, you won't be able to focus on any blue light further than 0.5m away! Not really a concern in the natural world but very noticeable in a server room full of blue LEDs or on a screen with blue text on black.
The difference in power between red and green is less but still useful enough for some vision tests. If you can focus on red traffic lights but green ones are blurry, this is why - consider a stronger power as long as it doesn't strain your near vision.
