Refractometre/PinPoint Salanity monitor/Milwalkee Conductivi

[Willito]

I am having a hard time deciding with of the 3 products to invest in. I want high accuracy, low maintainance, easy to use, and bang the buck!
Of the 3, which is least needed to be calibrated on a regular basis? Please, anyone with personal experience, give me your opinion. Thx.

Will

by the way:
Bue refractometre - $42
Pinpoint Salanity monitor - $115
Milwalkee conductivity - $70