I don't take much note on marine salt. Been using Coralife all these while and I don't get problem with them. I like the t-shirt thou, bright colour and fit me well.
After wearing sometime, it become my delicated aquarium rug
Redsea salt is value for money since they are more salty than the other brand of salt. You need lesser amount to get the same salinity.
I don't understand why you have problem dissolving these 2 brand of salt.
If your salt is fresh, as in they are smooth and powdery like "Johnson baby powder", they are so easily dissolved when mixed.
But if your salt lumps into big or small pieces of rock like structure, due to contact with atmoshperic moisture, you have a hard time dissolving them.
FYI, any salt mix that comes in contact with moisture in the air will start to lump up and form "rock-salt". Those are really hard to dissolve.