Before you plunk down your credit card, rent either one for a day and see how you like it. It beats spending the full retail price on the lens only to be faced with buyer's remorse if it doesn't work for you.
Shooting with supertelephoto zooms demands exacting technique at the long end. Any vibration or shake will be magnified and the image will suffer as a result.
Part of that technique means you must be able to balance, focus and trip the shutter as smoothly as possible. One of these lenses might be easier for you to handle than the other, and there's no way to predict which one you may prefer.
Long/heavy lenses mounted to your camera often make it front-heavy, as if the front of the lens were constantly being pulled down. I'm thinking the lighter lens is the one to start with to see how it balances for you. Then if you're OK with it, you can decide to purchase or try the other lens if you need the weather sealing and other features.