I have not found anything recent on the comparison between these two, so I'm going to work through the math here. Let me know if you find any issues.
Your buffed crit is C and your buffed crit damage is D. C is listed as your crit (spell / damage crit). D is listed under Advanced Stats, and is the listed Critical Damage % plus 50%, which is your base crit damage. The below calculations assume you're wearing 7 divines.
The Thief Mundus gives you an extra 9.1% crit. So, with the Thief Mundus C% + 9.1% of the time you will do damage 1+D, and (1 - (C%+9.1%)) of the time you'll do damage 1. So your damage number will be (C+9.1)(1+D) + 1 - C - 9.1 = C + CD + 9.1 + 9.1D + 1 - C - 9.1.
The Shadow Mundus gives you an extra 17% crit damage. So, with the Shadow Mundus C% of the time, you'll do damage 1+D+17%, and 1-C% of the time you'll do damage 1. So your damage number will be C (1+D+0.17) + (1-C) = C + CD + 0.17C + 1 - C.
The Thief Mundus is better than the Shadow Mundus for DPS if C + CD + 9.1 + 9.1D + 1 - C - 9.1 > C + CD + 0.17C + 1 - C ---> 9.1D > 0.17C
Dividing both sides by 0.17, when C < 53.53D, the Thief Mundus is better than the Shadow Mundus for DPS.
As an example, my MagSorc has buffed C of 55.2% and D is 70%. 55.2% is not less than 53.53 (0.7) = 37.47, so the Shadow Mundus is better for DPS.
If you calculate the total damage numbers, you'll find that they are actually really close, so I'm guessing you might not see a huge difference in your DPS between the two. One advantage for choosing the Thief over the Shadow, even if your DPS is slightly lower with the Thief, is that if you heal when you do crit damage (e.g., with Crit Surge skill if you're a Sorc), you'll get more healing with the Thief than the Shadow.
Are my calculations above correct? If not, please point out the mistakes. Thank you!