I think the one on the left, the darker one.

(you'll have to decide which gray tone works best w/your wall color)

although, see my last few photos. consider doing a wood color instead of the gray.

And, take into account the pattern you're picking. a pattern on one plank it doesn't seem like much. but when you have an entire floor of it, it's going to look very busy. I think the lighter one on the right is busy looking. You want a plank w/similar looking patterns on each one. (normally the pictures they use to print are 3 or 5 different patterns)

something like these first two are just too busy looking

this is better

However, gray can be very cold looking, especially in a basement.

don't disregard a nice wood tone. it looks great w/black and the gray walls, and the color will always be in style.

Gray is on the way out and your flooring is going to be around for 10-20 years. choose wisely
this one is nice. even has some slight gray streaks in it. Plus, if you ever want to paint the walls another color, you can do just about any shade and not be guided by the gray flooring.

here's a basement w/gray walls, and black fireplace. this flooring is busy and rustic looking, but it does have the warmer wood tones in addition to the grays.