It's definitely worth the trip. THen again, I'm one of these people who'll go to London for the week.
The nice thing as you pointed out is the small time change. In fact, it works to your benefit to stay on your schedule. They eat late there and you'll be right on time with the locals.
Having been to both several times, if you were going to choose only one, I'd say go to Buenos Aires. Santiago is nice but i find it to a lot more calm and placid than Buenos Aires. I absolutely love the energy that Buenos Aires seems to emanate.
I don't speak a lot of spanish and get along fine in both places.