Either way, just spray some frame saver in it and don't worry, be happy! Carbon and Aluminum advocates like to point ar steel rusting but the fact is that it only becomes an issue over decades of time even if you leave it sitting outside all the time. Rain rides and dry parking won't rust it except in a VERY long span of time. Remember - steel was essentially the only option for a century.
All the Shimano stuff is pretty comparable and all pretty high quality. Differences are pretty subtle. XT is extremely nice, Tiagra just plain good.