AP and Crit

So I have been thinking about the upcoming changes in 3.1. I currently sit at about 35K armor, 43K health, 45% dodge, 40% crit and close to 6K AP raid buffed. I know that my armor and health will go down but that my crit and AP will become a lot more important.

So if I am tanking multiple mobs, I know that I will crit enough to keep Savage Defense (SD) up most of the time, and that it would be a 1500 HP shield so that is nice. However, on a single target boss, will 1500 be enough?

If I change out my agility gems for strength gems, am I gaining or losing? My SD shield would be bigger, but my chance to crit would be smaller. So my question becomes, do I want a smaller more frequent shield, or a larger less often shield?

My personal thoughts on this is smaller more frequent. Since the SD shield goes away as soon as you take damage, you want to make sure you have one up as often as possible, therefore, you want it as frequent as you can.

If for instance you have 6000 AP and 40% crit, if a single Boss hits for 5000 per attack (100 attacks per fight, no dodges or misses), and assuming 40% crit means you will have a shield 40% of the time, your damage taken is

5000 x 60 = 300,000 (non shielded)

3500 x 40 = 140,000 (shielded)

for 440, 000 total damage taken

If for instance you have 8000 AP and 25% crit, if a single Boss hits for 5000 per attack (100 attacks per fight, no dodges or misses), and assuming 25% crit means you will have a shield 25% of the time, your damage taken is

5000 x 75 = 375,000 (non shielded)

3000 x 25 = 75,000 (shielded)

for 450, 000 total damage taken

However, from a very generalized look, it doesn't seem to matter much whether you have a large shield with smaller crit or a small shield with larger crit. It seems to average out in the long run. It will be interesting to test this theory and if I have time, I will go to the PTR and play around with different amounts of AP/Crit and see what I get.