Given the choice, I'd rather have a faster lens than a weather sealed esp if the body isn't sealed either.
Ask yourself If you are person who looks after their other electronic gear such mobile phone, iPad, lap top etc etc do you end up replacing them constantly? As long as you (reasonably) take care of 17-55 you should be ok.
This is a really good point. Too often we see a lens knocked because of 'build quality' issues or lack of weather sealing. Unless you need to shoot in poor weather conditions or are particularly hard on your gear it's just not necessary. You should go for the lens that gives the best results for what you need.
For crop I'd choose one of canon 17-55, sigma 18-35 or 15-85.