Not too familiar with big water fishing but it seems that you would want the heavier line for whatever wind there is. A while back I was in Wyoming chasing the Cutt-slam. The Colorado River's were is a small creek, maybe 6' wide. My 3 wt was fine, although a tad undergunned for larger fish but for Bonnevilles I used my 5 as it was windier (not bad, but breezy) and the 3 wouldn't have covered the width.