You can always take more off, but you can't add length back. If you hem your dress pants to little or no break, I would definitely go 31.
Tonight I'm going to fix two issue with the NS that have been bothering me for a while: flopping stacks at the bottom that have never formed correctly and darkness. Going to add a cup or so of bleach to the wash then dry with a thick cylinder in each leg to form the stacks around. We'll see how it goes!
Btw, I asked in the other thread but didn't get a response. Can someone tell me if appropriately sized W&H straight fits have a bigger thigh than NS? I like the ns but the thigh is still pretty tight on mine after wearing for about 6 months.