The waist on the 30s will stretch, probably to the point that they'll be hanging off your hips. So will the thighs and knees, and basically anywhere that starts off tight or where there's any pressure against the denim. If the thighs are tight now, they'll be fitted but a little loose after you've worn them for a while. Generally, though, the leg silhouette won't change too tremendously, so the question is whether you're happy with how the legs *look* right now, realizing that they will stretch enough so that they'll probably *feel* fairly comfortable in just a short while. If the 31s are already a little loose in the waist, I'd probably steer clear unless you want them to really hang off the hips. After a couple weeks, you'll probably need a belt to keep them from falling right off. This is fine if you want to wear them this way--I think LA Guy wears his New Standards like this, making it a loose-fit jean. As for shrinking, they'll shrink back down a little after washing, but a) they won't shrink (in the waist/thighs) any smaller than they already are; and b) they'll stretch right back out again in short order. Probably even quicker than they did the first time. Plus, you can stretch the waist while wet just like you can stretch the inseam (though to a lesser degree), if you're worried about them tightening up too much. My vote is for the 30s, unless you really don't like the leg silhouette.