I'm not sure what others will think, but I would go with the porcelain. There is a lot going on with the floors and countertops, and the porcelain looks a bit "calmer". That's just my $.02. Good Luck!

There is already a lot of pattern on the floor and in the countertop. I wouldn't add another pattern with the glass mosaic tile. You need a plain backsplash. The porcelain tile sample you show is too gray for the warm tones of the counter. I also wouldn't do a backsplash with different tile sizes. Too much already going on. If the floor wasn't so busy I would suggest a slate tile to tie the tones of the floor and counter together but I wouldn't use that with that floor. I would just do a plain subway in a white that matches the appliances and a white or off white grout. Another color there is adding another element without tying together what's already there. I would then add some accessories that tie in the floor color.

You're probably right about the busy-ness of the tile. I think my camera made it gray-er. It's quite a bit lighter in real life. I was trying to put a similarly toned color (blue) in the space because I am so sick of that green & white tile.
So if I go with the porcelain, should I go with 12" pieces or subway? I hadn't planned on different sizes of tile.

I should have thought about the countertop more. It's so ugly when right next to the floor in my pic!

I'd use the porcelain but cut it in half so you'd have 6x12 subway like pieces.

I was thinking blue for the wall color since I was leaning towards the glass, but now that I have seen the light, what would you suggest as a wall color for the center and left walls?
The rest of the space is all open and has white walls. Was hoping not to paint because it would be such a big endeavor. Since there's very little natural light down there, I'd like to keep it bright.
Thoughts?

I would not go with the glass mosaic. It is too busy as mentioned and I don't think the colors work. The porcelain doesn't go well with the countertop, at least in the photo. I would use an off white to light tan subway tile that coordinates with the counters and cabinets and brightens the backsplash. Pull all of that area together. I think that you want to downplay the backsplash and let the floor tile stand on its own.

Here is a link that might be useful: kitchen interiopr design

Picking a backsplash that matches the counter is the most difficult decision. I'm not happy with either of the choices but it is hard to tell from pictures. I would go with something on the cream side of the countertop.