I bought one over in NY last year at a dept. store (Fortunoffs had a great sale on at the time). When we got it valued over here, the retail price turned out to be almost 3 times the price we paid in NY, so once you get a quality certified product, it is certainly a good way to go.
The main advice I would give is do all your planning/sizing/window shopping etc. here in advance, so that by the time you get over there you know exactly the kind of ring you want. Each shop only has a limited amount of stock on any give day, so if you know what you want, it will help you quickly move on to the next shop if they don't have what you are looking for.
On the Tiffany's thing, yes, there is a premium. However, some people like the idea that they got the ring (+ box!) from there... that can be priceless to some people.