I would say that Sacrament 5 is a better choice, since that will give Last Rites (lvl 5) and Sacrament the same duration, making it less of a hassle to rebuff (if only somewhat).
The priest circles are filled with abilities that offer more then maxing out Sacrament or Blessing for a few points of extra damage.
Note that the increase to damage below 40-50% is almost none-existant. In my testing it appears to increase Sacrament bonus damage by about 50%…but only on the Last Rites damage line.
Which at a Sacrament level of 5, gave me [~50% of 24] = + 12 more damage. (on just the Last Rites damage line)
This tends to be the standard choice of skill point distribution. The difference in damage isnt much, but you have a lot more points to spend on healing and what not.