I'm nowhere close to being an expert, but my guess is that it depends on the skill level of the jeweler who would be creating the custom setting. Is it a simple solitaire, a halo, or some other kind of setting? Experts here generally are not thrilled with Blue Nile settings, but they also decry pave (particularly halo) settings created by jewelers who are not skilled at pave.
I'm hoping this response bumps your post up so that someone who knows what he or she is talking about will see it and chime in.
Depends on your diamond; some shapes are easier to set than others. Rounds are easiest to set in a standard mass produced setting (branded and unbranded). Fancy shaped diamonds like pears and marquises aren't always a calibrated size and tends to benefit from a custom fit setting. The above advice is for a standard solitaire.
If going with a halo or other design, going custom also allows the customer to select the proportion that he/she prefers (bigger or smaller melees, type of pave, etc).
The advantage of hand forged is that it is heirloom quality meaning it will wear better and last longer and will be in great shape to hand down.
The difference is that they do not use a mold they do it all by hand, it makes for a sharper design where detail is concerned. Using a mold causes impurities in the metal due to the process of removing the ring from the mold and it isn't as durable because it can become porous. It doesn't mean non hand forged isn't good just not the same.
Most hand forged platinum ring makes use a special formulation of 90% platinum 10% iridium which is stronger and better wearing than say a Blue Nile 950PT. That 5% is usually lower quality metals.
The hand made for a specific diamond will not have any gaps.
I always laugh when I see Blue Niles's recently purchased rings and someone has a $38,000 diamond in a $500 setting.
I'm not an expert just someone who has been in the same boat.
I think both choices are fine, but I love the 900PT!
Mark Morrell and Victor Canera both use 900PT and are very well known here on Pricescope.
CAD and CAST custom can be heirloom quality. I know my own CAD pieces certainly are.
For some pieces handforged is a complete and total waste of money. Just like a hammer, it's not the right tool for every job. For pave, it's the best tool. For other designs it is NOT.
CAD is superior for some designs to handforged as well. And then there is die struck manufactured pieces, etc which are also fantastic quality for some designs.
Blue Nile's settings have been taking hits recently, and no I would not buy a stock setting from them. But I would buy one from Vatche, or from Sholdt, or from BGD for example. And my BGD pieces have appraised to be of similar quality to Tiffany.
Quality of manufacturing is important for ALL pieces. But especially for engagement rings that get worn every day for a lifetime.
But you don't have to go custom or handforged to get great quality.
If you want great quality. Show us what style setting you want, and we can show you the range of options, and the cost of those options and help you come to the right choice for you.
This isn't a one size fits all scenario. And sometimes stock is the best quality, over custom. It all depends on what you want.