head pressure is more important when there's more things that causes pressure drop - high-resistant blocks, fittings & adapters etc... But I find flow rate is more important, especially now that my loop is much more simplified. D5 is more durable than DDC (from what I can gather, plus it doesn't need a heatsink to raise endurance), plus it's quieter also. Though D5 is really not meant for SFF, most sub-20L don't really have proper support or clearance for it except the Winter One.
why not utilise the T1 2x passthrough for external rads (Watercool MORA or Alphacool ones) in addition to your 1 internal rad? I've seen a few people done it and with a couple of DDC you still retain the mobility if and when necessary