I would recommend the phosphorus but it really depends on your desired level and acceptable range. The phosphate checker has a error margin of +- .04, so a reading of 0 could be .04 and a reading of .04 could be from 0 to .08. These are all ppt
The phosphorus is .03 ppm.
That sir is entirely your decision. Depends on your budget and need for accuracy with the measurement. It is called cost/benefit analysis and everyone has do to it for themselves.
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