I was reading about ExtractMojo and the refractometer that is used with it.

It seems that the whole point of the refractometer is to find the total dissolved solids, which is also what a TDS Meter does. A TDS meter is typically used for water purity where refractometers are used for wine and, in this case, coffee.

I know they use different methods to find the TDS, but what is actually the difference, for coffee applications, between the 2 of these? Does a TDS meter have an unacceptably low resolution or something?

the precision is important, most refractometer you can find for sugar, wine, etc are precise to 0,2% and the mojo is precise to 0,1%.

This makes a big difference due to the 18%-22% dissolved solids range

here's an article on the subject:
http://www.espressoschool.com.au/blog/4...xtractmojo

edit: +1 on tds meter http://theotherblackstuff.ie/guides/tedious-meters/