Doug,

I think I would get the heavier one for your 6 weight, but you could go either way. How deep is your typical water you are going to fish? If it is less than 4', you might want to go with the 5/6. Both should cast fine, I have cast mine on my 9'-6" 6wt XP and it did fine didn't feel that overlined, but seems better suited for my 7 wt XP. Since it is a constant diameter, and very thin, being a rod weight up or down is not going to be a big difference in casting effort. But I would not go much more than one wt off. Good luck.