I'd also recommend against XFlow unless there's some tube run issue that it solves. With the full-size DDC and D5 options available in this case, you should have no issue with restrictive flow. I plan on putting two HWLabs 280GTS Rads in it with an EK D5 PWM pump.
As for tubing, 10/13 seems to be the most often used tube size for soft tubing, although I use 10/16 ZMT from EK (matte black rubber rather than clear).
Just make sure you buy fittings that are correct, and note that Hardline fittings are different than soft tubing fittings. So if you switch tubing later, you will need to switch fittings.