-34s are great for use with T34s etc. when you want to really ring their neck for as much boost as you can get out of them through the rev range!

T4 turbos however want to make boost for fun, normally you end up having so much going through the wastegate at high revs that you can encounter boost creep when the wastegate isn't big enough! As such, a -31 is better for helping to regulate the boost as you don't want to be trying to hold the wastegate shut so much, especially as you're not trying to run 30+ psi.

Just my take on things