Usually I prefer subway tile but I really like the smaller tile backsplash and think it adds interest and texture.

The 3x6 subway tile. Its classic and has fewer grout joints to clean. The color is gorgeous!

Both go and are beautiful, but I love the larger subway tile. Very clean, simple beauty. You can add pops of color with accessories. Over time it will never be outdated.

I feel that with the Frosty Carrina countertops the Bossy Gray 3x6 would be a great choice. The marble tiles may compete with the countertops a bit. The color is beautiful and will compliment the veining in your countertop nicely. Also, plain subway tiles are simple and elegant.

Go with the mosaics. Adds more interest and will keep the kitchen from becoming too monotone and boring.

I like the mosaic as well. Don't worry about the grout unless you are a particularly messy cook. The subway is pretty too and the glaze variance is interesting detail.

If you can't decide, do both! Use the subway tile for most, and add a feature with the mosaic. Most people put a mosaic behind the stove. Put a square of the marble mosaic, of the size you want, run a frame of the subway tile around it, with mite red corners, like a picture frame, subway tile the rest. You may even find a matching border tile.

The Sonoma subway tile. Frosty Carrina has a marble look so I wouldn't pair it with real marble for a couple of reasons--competing patterns and making the frosty carrina look like fake marble.

From my vary recent experience you can mix Frosty Carrina with marble. I just installed volakas marble tile with Frosty Carrina and I absolutely love it!

I think both of your options are great! If your cabinets are like a warm white, I think milky way marble is a beautiful choice. (I have samples of milky way too, it was one of my top choices, but decided to go with something with more veining)

I like Rachuba's idea, use both. Many designs to think of: use subway tile with a 2" or 3" band running horizontally through the tile. Or use the marble, inserting a band of subway tile. Another thought, run subway vertically, stacking I think it is called, with a narrow band of marble running vertically every 3-6 subway tiles.

Mosaic, it will add some interest to your cabinets.

Sonoma, for sure.

Which you choose depends on where you want to draw the eye - Choosing a kitchen focal point? Here is a great
article that might help: Kitchen
Focal Point