Ok,
Tiny essay, then ive an engine to build....

The -31 has a base lift pressure of approx 8psi
The -34 has a base lift pressure of approx 14psi

The -31 and -34 springs are also linear/progressive rated.

In a scenario where you require 20psi with no form of boost control at all, a -34 is best as the -31 becomes useless after 15psi achieved via spring preload.

EG:
With a base lift of 8psi, to make her lift at 15psi would require you to compress the spring enough to make it lift at 15. A -34 will be in a minimum loading situation at this pressure.

However,
With correct boost control applied, we simply dont send 15psi to the actuator to make it open at 15psi, we BLEED enough air to the can to match its spring load setting.

IE:
We want to run 20.
So we load the -31 to say.. 10, and we bleed 10psi off via amal or bleed control, so the actuator ONLY gets to see 10psi.

Now,
In cases of extreme turbine housing pressure (T34 at 24+psi is a good example) we can hit the scenario where the turbine housing pressure alone is enough to overcome our spring. This is when the -34 comes into it own.

Hope this helps..