Is it better to get identical computers and be able to failover between consistent devices and algorithms? Or is it better to have two different watches and observe which is more conservative during a particular dive?
If you are starting from scratch, I would recommend keeping the same algorithm and consistent devices.
I say that because your second statement:
Or of course, if the different computers are diverging in their guidance, clearly one of them may be suffering a malfunction.
...is not necessarily true.
IF there is a malfunction, then the two may differ wildly. But the most common divergence is because they have different algorithms. If one is a proprietary algorithm, then you may not know why you are seeing what your are seeing. The less likely reason would be a malfunction.
I have dived SP's algorithm for years via the Luna. A good solid computer which (at MB Level 3) suits my conservatism nicely. Nice displays. Logical.
That said, I've converted to Shearwater for two reasons. First, the implementation of the algorithm is fairly transparent, which generally doesn't matter in rec diving.
More important, the ability to see the bloom in your tissue saturation in the last 20 feet is, I am convinced, a potential safety area that in time I think will become a real area of focus. Personally, having watched what I see on my Shearwater, I am now slowing my final ascent from the safety stop markedly, even on rec dives, because of what I saw and what Scubaboard has provided by way of discussion.
It's a new frontier, and it may not make any safety difference whatsoever. But I think it is significant, especially for repetitive dives, for dives that go near NDL, or as you age.
My 2¢.