I went with the 'cheap' one that cost around $25 or so. They must've come out with a newer version for $29 which looks even better than mine. http://www.airwaterice.com/product/SENNODUALTDSMETER/Dual_TDS_Meter_Senno.html
There's no need for the industrial one, you can always call and talk to them about it, I've had excellent customer service from them and have bought their equipment/ filters before.