You didn't post a link but I assume its the HM Digital DM-1 dual inline.
I have a couple of them but much prefer a good handheld for several reasons. One is the inlines are not really temperature compensated so can be off unless your air temperature and water temperature are not exactly the same which never happens. Another is they cannot be used portable so are dedicated to two points, usually post RO and post DI so you cannot test tap water, ATO container water, the LFS water, bottled water, loan it to your buddy etc. I like to know what my tap is as well as the RO only so I can calculate my rejection rate then check my DI TDS and also the water in my top off container so the handheld is much more convienient and more accurate, especially on the low end like RO and RO/DI.