Too late to jump in this I assume but I will speak up here for NSW.
Before I do that, let me put a disclaimer here.
You go with whatever works for you and is convenient/ cost effective to you.
With that said, depending on where you are, NSW is cheaper then FSW.
Pet stores sell here for 49c per gallon compared to 59c per gallon of IO.
If you make your own fsw with io it will be cheaper but if you were to go with high cost name brand then it will be more expensive then nsw. For example, one that Seth is ordering is pretty expensive and if you do gallon by gallon comparison from that it will be equal to or cheaper. (you mentioned ***** so I'm assuming that you are buying your water. )
We the hobbyists are trying to simulate ocean and stimulate our livestock to think that they are in the ocean. If you have an option to get mother nature why not go with it?
Yes, you will get less mag/cal rate then synthetic but fsw also contain preservatives that are at times not so great for our corals and I know for sure you will definitely have to dose for IO.
By the way, corals which all come from oceans live well under those condition, key here is that you replenish whatever corals eat up and do it consistently.
Consistency in this hobby is the key. (salinity, temperature, ph, alk, cal, mag, nitrate, nitrite, ammonia, p04 etc etc)
I use NSW and dose 2 part doser from seachem and dose a cap full of each everyday and honestly my cal is never below 460 and parameters are all in check.
It also comes down to whether or not that company is reputable company and that they are consistent.
If you collect near shores then you are out of luck. We have piers that stretches over couple hundred fts and have inlet pump and it's no boat zone and that place is pretty much safe.
Person that I go with, always get it from from high incoming tide and only if when the weather is fine and run uv on it for 24-48 hours to kill any free floating particles.
I have used it for 6 months or so with great success and my corals and my fish are both thriving.
I know numerous amount of people who have beautiful sps tanks using nsw. ( calcium is dosed ofcourse)
I have tested on numerous occasions and I have consistently received the same numbers and once again consistency in this hobby is the key.
Why do I keep mentioning consistency? Because FSW is man made and no man made is 100% consistent and there are many horror stories of bad batch of FSW and NSW is close to none.
But once again this is only my opinion so please listen to it with grain of salt and make your opinions and decisions based up on your research and go with whatever is convinient for you.
I use IO reefcrystal and Red Sea Coral plus, time to time and it works great too just wanted you and potential other readers better information on NSW.
BTW, go check out FloridaJoe and my tank and tell me that nsw doesn't work.