I have owned a Juice 6.9 and own an I3 222. I do not think that either manufacturer is marketing the correct paddler weight for the boat size.
With each boat, paddle one size smaller than what you would expect. I weigh 205# and had a blast in each. Then seat needs to be moved as far forward as possible in each boat otherwise you will be staring at the sky. Both boats played well and ran the river well. They are very similar shape and specs. The reason I sold the Juice was I could not get comfortable. In Dagger boats I feel that my knees are too high, pinching my cyatic nerve and making it harder to set-up for my roll. The Pyranha osition has my legs straightened out a little more. I know the I3 and I4 are a little different (~5gals increased volume in the I4 (I think they increased the stern volume because no one was moving the seat far enough forward)), but they will essentially paddle the same. I paddle my I3 in class IV and take it to the park as well. Demo each and see which feels best for you!