Both destinations for our trip last year was great. Thanks to Gaucho100K for good tips in our planning.
We spent 4 nights in BA and one night in Iguaza Falls of which if you can get there in day and fly out late the second day, you can do a lot of the fun things it has to offer. I would highly recommend to somehow to incorporate that into your trip, it was absolutely breath taking.
Santiago for 5 nights was very fun and safe. I did 'feel' safer walking around there than BA, but overall BA was still great. Did day trips to Andes, Valipraso and Con Cho Toro (spelling
).
I don't think you can wrong with either destination with lots to do in both locations.