If you're going to fit Hiwin style rails the correct choice is the one that's easiest to retrofit to your existing plates if possible at all.

15mm Hiwin are a PITA to fit with 1605 ballscrew and BK12 fittings you need spacers, I fit my ballscrew sideways on, on my Y axis to solve this problem but still ended up with spacers on the X & Z.

15mm was a no brainer for me, even with the extra hassles but I traded time for my original Hiwin's in the end cost me 12 hours on a Sunday and 6 hours in the week for my 1.2m Hiwin's and 45cm ones on my Z. Was it a good deal? Like my friend tells me my time is worth nothing lol.