given that you will not be doing far off tours for possibly a long time, the 520 will be more fun to ride, as sti's are just plain fun to use. The sora level 9 speed shifters work and feel just like the previous gen of tiagra 9sp shifters, ie the trickle down effect, and will last a long time iin real life. My tiagras are 9 years old and still shift perfectly well, and have ridden newer soras with the paddle shifters and they feel identical.
re alivio stuff, sure its lower than deore etc , but it will last well also, especially if you use common sense and keep your drivetrain clean.
Its an obvious price point that they didnt use deore 9 sp 48/36/26 crankset, although that could also be because that crankset now is officially 10 spd., so slightly heavier but will still work.

re riding in winter, its still a nice bike, so if you ride it in salty conditions, youre not doing your bike a favour....get a crap bike for salty conditions, it aint worth it.

oh, and backpacking is using frame bags and whatnot, not panniers.
enjoy whatever you get young feller.