I prefer the one on the left.

My preference is the left one.

So both are same shape (except one is shown turned differently than other). Looks like one is larger than the other? And it's a natural product so you are going to have variances in veining, color, etc and the purchased tile may not even look like what your samples show thought the more expensive one might have something to do with the veining. Is there anything else that we should take into account? Like the size of the bathroom where one size tile might be better than the other?

As they stand I'm guessing the left one is more expensive but that shouldn't determine what way you go unless your budget is dictating it.

So- the one on the left around $8 a sq foot and the one on the right is $21. They're both 2 inch hex.
The savings are substantial but I worry about the less expensive one as it's from online retailer "stone center online". I'm torn because I see as many negative reviews as positive. The cheaper one is a bit thinner. Any insight regarding that factor being negative?

If you're going to have a pro install it, ask the pro to check both samples. Whether you buy "online" or from a actual tile store there will probably be quite a bit of variation in color. Make sure that the retailer will accept and refund for any tile you need to return...then buy at least 50% more than you need so you can cherry pick through them (cracked/chipped/bad shading).

where are they from? manufactured? what type of carrara are they? the one on the right looks a bit whiter so maybe that's why, along w/the thickness, it's more. I have heard of some of the carrara from Floor and Decor having issues because it's a lesser quality marble manufactured in China. There is Statuario , Bianca Venatto, calacatta, ordinary white, bardiglio,etc,,,,these types all carry different price points. can you tell me where you got these?

I was going to say they look very similar but now that you said the right one is significantly more expensive it instantly became more attractive