Glossy can be more slippery is the first thing that comes to my mind. I do personally love a matte. Darker grout or no? It depends on what you have planned for the bath in terms of vanity and the rest. There will be a lot of grout lines so a darker would be a good option if it works well with your selections.

Glossy does not mean more slippery necessarily - it all depends on what the tile is made of and what its moisture absorption and coefficient of friction is.

Matte:

https://www.homedepot.com/p/Merola-Tile-Metro-Penny-Matte-White-9-3-4-in-x-11-1-2-in-x-6-mm-Porcelain-Mosaic-Tile-8-sq-ft-case-FDXMPMW/202647813

Glossy:

https://www.homedepot.com/p/Merola-Tile-Metro-Penny-Glossy-White-9-3-4-in-x-11-1-2-in-x-6-mm-Porcelain-Mosaic-Tile-8-sq-ft-case-FXLMPW/100654083

For very small tiles such as these an expert setter is required. Nothing worse than seeing irregular grout line "gaps".

That is very pretty too. Is it 1” or 2”?

Matte is vintage. Is that the look you are going for? Glossy is not period appropriate and it is also slipper, though with many grout lines, that effect is lessened a bit. I would prefer matte on the floor.

Either gloss or matte or will work with that size mesh mounted tile. The abundance of grout joints is where you will get your traction.

I have been enamored of penny tile for the past few months but I think hexagon is a better choice to avoid layout and cleaning issues. I think it's also hard to get a clean cut on a penny because you can't use a tile saw. Experts can correct me if I'm wrong.

@mayflowers - Yes you can use them on a wet saw...you just have to be very careful. They also make tools specifically to snip smaller tiles.
As for cleaning grout (for tile, not natural stone) use vinegar and water, 50/50 solution. If you have natural stone, 1 cap full of Dawn dish soap to 1 gallon of water.
You can also use an upgraded grout for the bathroom floor. Most upgraded grouts do not require sealing as they do not allow moisture to penetrate. They normally cost more as well. -- Do not use an upgraded grout for the wet floor of the shower, this are needs to breathe properly and standard sanded grout should be used.

I have seen the snips but the results weren't a clean cut in the photos I saw. Someone posted a few days ago with sloppy cuts around a shower drain. The faint lines between the sheets concern me for a floor layout. Corners where shower accent tiles meet don't look great either. I like the small hexes just as much and I'd rather not take my chances with pennies, but that's because I'm a perfectionist.

Seems like a square drain cover would be a good suggestion for many shower tile installations.

Have you looked at a Hex style tile? I would be using hex instead of the penny tiles myself. I would also do it an a Matte.

Both glossy and matte are attractive, I don't think it makes all that much of a difference with those small tiles on the floor. Matte is a neutral look, glossy a bit more "glamour".

I read about using the snips on the Young House Love blog. Is this the best results you can expect from penny tile? I wouldn't be happy with that in a visible area. On a floor the rough cuts would most likely be covered with base molding.

Wet feet are the issue. I'd want matte for the shower floor, but you're going to step out of the shower onto a mat or rug so I wouldn't be too concerned about using a glossy tile on the floor. It wouldn't be any different than walking on porcelain tile in a larger format.

https://www.younghouselove.com/backsplash-bid-ness/

Love the penny tile!

Can you imagine spending $65 for something that hair catches on? I mean really, do we need to decorate everything?

We imagine that many home owners are spending thousands of dollars on tile and fixtures then getting stuck with a home depot cheapo drain. Our clients love the fact the drain helps keep the drain clog free!

Love the drain. Can you retro fit it into a tub shower combo?