i won't consider weight a real problem in this scenario, any of those systems should be more or less the same weight, what i think it really matters is ergonomics. The deal is to find a good balance between what is a must to you (prism? motordrive?) and the weight you would carry, i can tell you for sure that a camera without prism and motordrive its gonna a be lighter! dont focus your attention to just weight, any of those cameras were not made to be extra light, and 300-500gr more or less is at least for me irrelevant.
I was in the same position as you a month ago and i can tell you that you can spend days searching for the perfect system or listening opinions.
Do a favor yourself, go to a shop, hold and play the systems of your choice, even ask them to rent or at least shoot a roll inside the shop to try features.
After trying them you should have a nice idea if that or this camera works for you, then you can buy it anywhere cheaper. And i can tell you that i needed 2 minutes to choose one between mamiya, pentax and contax.