Here are the calculations for determining which is better.
Insightful Earthsiege Diamond
- +21 Int = 24 MP5 for Discipline and 17 MP5 for Holy
- about +80 MP5 from proc effect = 80 MP5 for both Discipline and Holy priests
- Total stat weight = 104 MP5 for Discipline and 97 for Holy
Ember Skyflare Diamond
- +25 SP = 50 MP5 for both Discipline and Holy priests
- +2% Int only grants about 30 Int for Disc and 24 Int for Holy. So equivalent to 34 MP5 stat weight for Discipline and 20 MP5 for Holy.
- Total stat weight = 84 for Discipline and 70 for Holy
So the Insightful is still far better than the Ember. This applies to both 3.1 and 3.2.
For the Ember to better than the Insightful, with these stat weightings, Discipline priests would need about 2420 fully buffed Intellect and Holy would need 2925 fully buffed.
Edit: BTW the numbers are based on these stat weightings.