Unless an 18 constitution somehow miraculously cures diabetes I think I'd go with charisma. An 18 charisma might help me break out of my shell a bit and help me meet people.
Then I'm definitely picking constitution. I've come to terms with it, but if I had the opportunity not have to constantly jab myself with needles I think I'd take it.I imagine it does! You can’t have a physical disease if you have 18 Con! The increase in ability score includes all relevant phycical changes - more muscles, less illness, whatever.