Upper Rustic is really funny. First time I ran it was at 4.2 or 4.3 I think, and the whole run including the White Mile was not very memorable. The next time I ran it was in 2011 above 5', and White Mile is huge at that level. That run really doesn't get going until 5'.
Lower Rustic is good above 4' in my opinion, with higher being better. Any lower than that it's not worth the extra gas to drive up there. The three rapids Grandpa's Gorge, Death by Dismemberment and Hole 99 are still fun below 4', but the rest of the run gets slow and flat.
At low flows (actually all flows - but at low flows it's more noticeable) Upper Mish is the better run.