I believe you have that backwards.
I’m almost certain that Sacred Strength (your “increases” option) is multiplicative with other damage increasing sources and thus has no diminishing returns. This is why the amount is so low.
Divine Purpose (your “more” option) does have diminishing returns in that it requires a GCD. Once/if we reach the point of being GCD capped the DP procs would have to push something else out to be used. That in itself is a form of diminishing returns because we would then lose the damage of whatever was pushed off the table and thus the value of the free proc would be much lower.
DP is better for now and there isn’t much debate because we are not GCD capped and free abilities work in easily. Will higher haste levels push us to the point to where SS overtakes it in later tiers? We don’t know yet, but its possible.