the last photo seems like it would just present a lot of ledges for slime and stuff to collect on. main difference between the first two is ease of grouting (skill and care required)...

You're wanting to create a 4" pebble tile border? Not gonna' work unless you plan to set each pebble by hand. Only flat cut pebbles, like the first pix, on the floor. Rounded pebbles are murder on bare feet.

At 4" it's too small of a border to do with either the flat or the sliced pebbles; think of how many cuts you would be looking at versus the stacked version. That said, color is something to think about as well, if your surrounding tiles are close in color to the lighter pebbles, the border edges bleed out and don't give a crisp edge visually. We've done the look, but always used a pencil above and below to create a defining edge between the materials.

There's too many choice to list; post photos of your selections so far and the space. On the pebbles you can consider doing a waterfall effect.

A light gray tile well chosen and no "border" is nice and clean and modern. What is the need to tart it up?

I think borders and accent strips are already out of style and as soon as you create it, it will look dated. If you find a mosaic tile that you really love, you can create an entire wall with it, such as an entire shower wall, tub wall, or wall above vanity behind mirrors. But that's $$$. If you don't want to do that, skip the accent tile entirely.

My daughter used flat pebbles in a border in one of her showers.

For a 4" border, the stacked will probably look best. The linear nature of the individual oval stones stacked upon one another will help carry the horizontal theme of the inset, and the stones can be set tightly to minimize grout.

The other two can physically work. But they'd likely be more efficient (more stone, less grout, better individual stone orientation) in an inset larger than 4". A thin inset with sheets of stone simply cut into 4" strips can look slightly jarring, the installation can look its best if each stone is peeled off the mesh and hand set.

With either of the rounded stones, have a conversation with the installer ahead of time, before setting and definitely before grouting. You never know what people are thinking, but you can have a better idea if you have a conversation first. What you don't want is a deep bed of grout, where you end up with itty-bitty peaks of rounded stone barely visible in a sea of grout. The nature of the inset strip being just 4" wide should help, as the thickness of grout in the stone inset should not be deeper than the grout in the wall tiles above and below the inset.

I hope that makes sense. Good luck with your installation.

One last thing...

I prefer odd numbers, and the stacked pebble installation as shown will be made up of individual rows of pebbles. So if going with the stacked pebbles, consider for example, five rows of pebbles within the inset instead of four or six.

It's 2019....................... )Fourteen years past 2005.....and the pebbles. : )In other words, how will you feel in another five or ten years?

Pebbles are not dated or trendy to me. They can be a very organic and zen type look. I live in So California and have seen them in various applications since I was a kid. I would not overuse them or turn them into “waterfalls” but a simple accent of pebble in a niche or border looks very tranquil and spa like. I would stay away from non-natural colors. Do what you like.