Personally, I would go with the coil. I'm a big fan of things that look clean, and, for me, the fewer fittings the better. But there are pros and cons to everything. The soft copper might be a little more expensive and you may not get a perfectly straight line, since it comes in a coil. With hard copper, you'll have the straight lines with perfect 90's and it may be a little cheaper, but any curves you may want won't look as good (think of the numbers 5 and 2 on a digital clock), in my opinion anyways. The fittings are the same, so there's no worry there, but the soft copper work better with Flared, but I think most water cooled systems use Compression anyways. I guess it comes down to if you want a more industrial (rigid) look or something that's clean, but somewhat elegant.
And as much as I like ASUS, sometimes it's hard to beat a tied-in color scheme in a well built rig. So I blame you not haha