Not sure, but I have understood that subway tile does not need any spacing at all if needed. But between those options I would go with 1/16, less is more :) I prefer a more seamless look.

I would go with 1/16". With the dark grout on white tile, it can be a little less forgiving than white grout, so it may be less obvious if something is a little off with a smaller grout joint. If the tile has little bump on the sides, it has built in spacers and you can just butt them together, as well.

I'd prefer 1/16 and I'd go lighter on the grout. The fatter grout will overpower that size of subway and look too busy in the dark gray. Maybe something like this?

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1/16 with that grout, for sure

Then go skinny for sure.

If your tile has built in bumpers as show in the photo below, butt the tiles side by side to create a 1/16" grout joint. If you choose to have a slightly wider grout joint, use pennies as spacers.

I am going to go against the grain here. If you are using a darker grout (anything but white) then the darker grout will show up every imperfection in the tile and it gets magnified by a smaller grout joint. I would go with the 1/8" for that reason.

They are lugs. If you look at the tile from the side you will see besides the lugs the tile itself is slanted towards the face starting from the lugs. This is what create the grout line. Also how the tile person actually grouts will later the size of the grout line.

If a tile person over cleans his grout the grout gets washed out of the larger portion of the space and into the narrower part of the "V". You can have the same tile spaced using the lugs and have different size grout lines. With todays waterproofing systems and advanced thinnest the grout is just decorative especially with butt installed tiles.

With darker grout look into Mapei Flexcolor grout. It has a sanded grout appearance and is more difficult to apply on vertical surfaces but it gives a very durable finish that has a dead on color that can even be touched up and still have the same color. Dark color cementitious grouts are notorious for color issues especially if the installer over washes or doesn't mix as per instructions.

The Flexcolor is premixed and the color is accurate throughout. There will be sand everywhere but in the end the extra effort is worth it. With the Flexcolor I would use 1/16th spacers but remember it will give you a larger grout line as the tile itself already has 1/16 built into it. The larger line allows the sanded type grout a space to get into. No it isn't regular sanded grout it isn't even sand it just has a sanded appearance and feel to it. 2 different products.

I'm going to agree with Brickwood Builders. Ceramic tiles aren't perfectly sized like a rectified porcelain would be. The dark color grout will show up every imperfection in the pattern.

Meant to say I'd use 1/8" grout for reasons stated, more forgiving of those imperfections.

Maybe ask your supplier if they think it will lay up very evenly with the 1/16 joint. That said, you mentioned you like the vintage look, so I'd still go with the 1/8.

When the tiles are butted side by side they will create the 1/16" grout joint. If you want a 1/8" grout joint, use 1/16" spacers.

The bigger the grout line the better for you if your skill level is not the best. Helps with the variations in material and mistakes made if any.

@Gregory Boccuti

I am trying to make the same decision you were a year ago - Subway tile (though ours is square), using a medium-darker grey grout for a sort of vintage feel, and don't know if I should do 1/16 or 1/8. What did you end up doing and would you share a photo?!