Good power phase, with my humble knowledge I can tell you is what makes your voltages are stable (so less vdrops & vdroops). Corriganme if I'm wrong. It is assumed that the more phases you have, you can leave it more stable voltages and therefore you can do BETTER OC. Influences both the plate with the psu. But you left PSU with a plate HX 1000 and, with these two options for the same. The UD5, will be best for his 20 VS 16 phases of the UD4. Same chip, go for the 20 without thinking, if it comes into your budget.