Thank you all for the input!
@popvulture, what cuts of 3Sixteen did you have? Any comparison notes to how the 555 fits?
@guiltyparty , incredibly helpful to know first hand the differences in denim characteristics.
@Germinal , incredible that 25oz wears so easily. I'll likely start my IH journey on a lighter fabric, and graduate to a higher weight class.
At this time, it looks like I'll choose between the 21oz indigo and 17 oz natural indigo (which seems to fade a bit more slowly). It seems from the photos to look similar shade of indigo, but that might just be the poor quality of my screen. I'll take closer look at the measurement charts and read through the respective threads on the forum. Thanks again!