(below edited from orginal post to remove information not relevant)
That’s a difficult question to answer.
I think the question/issue is how to comparison shop (apples to apples) between the diamonds listed with the vendors here on PS and those at a local B&M stores.
Also with regards to price - you do not pay sales taxes if you buy from a web vendor outside your state (ahem ----but of course you should claim this on your IRS statement – right?).
Bottom line – I think a local jeweler could be competitive but I don’t think they would be lower cost. However, if you know exactly what your specs are you can go into a local store and give him/her a chance to meet specs and the price you found online.
Oh - did not know you were talking about that type of diamond. Thats why I stated that I was no expert. I do not believe those reports work for your shape.
I spent a lot of time looking in the NYC Diamond District for a stone. I found that it was difficult to get any information other than what was on the GIA report and that is insufficient because it does not give specifics as to cut.
A very well known dealer in the Diamond District said he had found the perfect stone for me and it was Ideal cut. I found Pricescope by accident and posted the table and depth of the stone. Several Pricescope members responded saying that the stone was too shallow and the table too large.It was far from Ideal proportions. There are a few Pricescope vendors in the Diamond District who have excellent reputations and you might try one of them. Diamonds by Lauren seems to be good on cushion cut as you can see from another post.
I contacted several of the vendors on Pricescope and found them to be very helpful. I wound up with a gorgeous ring from Whiteflash at a great price. I bought a round stone but I have noted that several people on Pricescope have bought cushion cut including Reena who seems to know a lot about them. Most of the out of town vendors will send a stone or stones to an independent appraiser where you can see the stone and decide if you want it. I did not pay for my ring until the day before it was ready to be shipped out by overnight Fed-Ex.
window shopper,
The answer is neither or both. No one has the best prices all the time on every stone. As a rule, you can find better prices at independent stores than malls. You can find better prices on PS than on Ebay. You can find better prices in a small pawn shop than a large one. Your neighbor can get divorced and sell you her diamond cheaper than anyone can match. You will be better off deciding what you want and finding a good deal on it that shopping for the best price. Price shoppers often end up with the stone that seemed like a good deal but really was not. Quality shoppers often get the right quality but overpay. Smart shoppers find both whether local or internet.
I bought a cushion from another store a few months ago. He was talked into buying it from a dealer, yet his customer base is not really into cushions. He has had it for 3 years. He needed money and sold it to me for his cost, which is cheaper than I could now buy it. I sold it in 3 days for less than I should have been able to sell it, everyone wins. If that same customer was so happy she wants another one, I can not match what I did for her. The business is not as simple as is seems sometimes.
You specifically asked abut NYC dealers and whether they should be considered a particularly good place to shop. Although there are many fine dealers in NYC, some of whom even have showrooms and offices in the diamond district, it's not their NYC addresses that makes them good. You will find that there are excellent sources elsewhere as well. It's a common mistake to assume that the source of diamond for the country, or even the world, is a few block radius in New York and that by shopping there you will be getting better diamonds for better prices. This is simply not the case dispite what the hawkers on 47th street will tell you.