I found out yesterday myself that Holy Prism is infact ALOT better than Execution Sentence ... but not forever .
It all started when I actually saw someone use it in a dungeon and was curious to how and why is it better .
I found the following formulas in Wowhead (punched in my own numbers and they were correct).
Holy Prism does : 1.428*Spellpower + 16136.5 (20 sec cooldown)
Execution Sentence does : 5.936*Spellpower + 12985 (60 sec cooldown)
Now before you start telling me "WoW look at the damage on Execution Sentence" you have to see the real numbers behind it .
If (and I say IF) you can cast Holy Prism on cooldown then it equals to 4.284*Spellpower +48409.5 (per minute) compared to Execution Sentence's damage it has ALOT higher base damage which means that now it does more damage until we aqcuire better gear .
The Spellpower breakpoint where Execution Sentence becomes equal to Holy Prism and then surpassing it is 21455. Which means you need 42910 Attack Power for that . Which means you will need a total of 21455 Strength to achieve that so you actually need 21268 strength (187 base strength from being level 90) from gear .
If you think I'm being helpful write something good in the comments . If not tell me why .