As you can see from this picture, the shields are comparable.
The Torto shield has more pierce. In fact, a regular Torto pierce shield has as much pierce def as a max pierce snarby shield.
Liewise, the snarby shield has more normal defense than the torto shield, probably a max UV's worth of difference.
Both give the same amount of health.
(The heat bonus pictured on the torto shield was a temp UV from the forge.)
I'll be using my new torto shields in beast levels that are shock or poison, and I'll give it a try in snarby shadow lair, which has a poison environment.
If you are wearing dragon helm and armor and have a 5* pierce sword that does at least medium damage vs. beasts, then you are already maxed out for damage vs beasts (or at least sword damage - you can still get a bit more out of your guns and bombs). In that case, the greater pierce defense found in the new torto shields might be preferable. especially in a poison or shock environment.
But Savage Tortoise has two status resistances, which are significantly more useful than the regular defense, as well as having a shield bash [that's actually useful].