I share the same view, my experience as follow
Sea water
@ Running cost is very low , 12-14LL / $, but
@ Damn heavy to carry. need transport and
@ Worry that, if it ever breaks along the way, there gone your car haha...
Mix salt water
@ Salt mix is more expensive
@ Not light actually.
@ Additional cost are DI/RO equipment , test kit etc..
@ Could be messy as you need to mix it
Anyway, it is my 2 cents worth, by the way, I always 03 sea water for 12 hours before adding into main tank. So far so good. Cheap, Simple and quick, need not to check parameters ..