Hey there.
Been living in Sydney for a few years and travelled around Boston a little bit when I was little (I was born in NH and have relos there). Was a while ago, though.
Both are beautiful harbor cities. Syd is great for the beach and outdoor events (though we've had a very odd and rainy summer) and its vibrant and multicultural. It's pretty easy to get around the city. The biggest drawback will be cost of living, especially rent. Don't know much about jobs after Uni, but immigration will certainly play a factor. Don't know specifically about your field, but you may luck out and be on the list of jobs underrepresented in Oz.
Sydney
In Boston you'll have four distinct seasons (and see New England's famous autumn foliage) and be reasonably close to NYC and other popular areas of New England, if you want to travel or look for work after school. Boston is also one of the more historical cities of the US, so there'll be plenty of Yankee Doodle stuff, if you're interested. Can't speak as well to cost of living, etc because I was a teen when I passed though. Although I do have positive memories. New England can be quaint in the best way, and I remember the city as being appealing.
Boston
Hope this helps.
