If the weather sealing is what you need then by all means go for it. I'm not judging, it's one of the basis I consider when buying equipment too. Since I live in the Seattle area I get a LOT of rain, plus I hike a lot and the waterfalls and dust I encounter I just don't want to have to worry about them. I completely understand wanting to spend the extra money on weather sealing
Keeping weight down is a major issue too. If you don't want/need the faster f/stop and feel the extra reach to 300mm is more valuable, by all means. It sounds like you've got your basis covered. You didn't really mention wanting to shoot landscapes, so I think 24mm should be wide enough for you. I adore my 17-40L, 17mm is SO WIDE on full frame, but normal daily shooting doesn't need 17mm.