Instant Ocean - the cheapest salt there is - is the best salt there is. Why? Because other salts don't do anything that IO doesn't, and IO is the cheapest. That means it's not only the cheapest, it's the best value as well. When many of the best tanks in the world use it, why would anyone think IO is not perfectly fine? (This is a general question and not directed at the OP)
As mentioned, the only reason to choose another salt would be to get something closer to the levels of your Big 3. However, that can be done much cheaper by either getting your levels to match IO, or adjusting the levels while mixing.
I understand some people might want to pay for the convenience of a different salt that more closely matches their Big 3, but I don't want to pay 2x or more the cost for that convenience. Just my preference.
If you do 10% weekly water changes on your 120, that's 624 gallons a year of saltwater you will need to make. Using a lab-grade conductivity probe, I've found that 156 grams of IO is required per gallon to get 35ppt salinity. For 624 gallons of saltwater, that requires 214.6lbs of IO per year, or 3.57 boxes of the 200 gallon (60lb.) boxes that cost $38 each. Your annual cost would be $136 (I'm rounding).
I've not used the Seachem, but if we assume the same weight required per gallon to get 35ppt salinity, you would need 4.25 boxes of the 220 gallon (50.5lb.) boxes. The cheapest I currently see that salt is $80.56 per box, or $342 per year.
It's possible the Seachem needs less weight than IO to mix to 35ppt, but I don't know. IO claims 200 gallons for their box (actual is 174 gallons at 35ppt), and Seachem claims 220 gallons for their box, but at 35ppt it probably makes less than 220 gallons.
So is it worth 2.5x the cost of the salt for Seachem? Not to me, but that's for you to decide.
Hopefully this helps in making your decision. Good luck!