What meter should I use!?

I am building a bubbleponics unit and everything has been taken care of except for the meter. I have a pH kit from general hydroponics but I want to be exact and plus you can't measure the nutrient mix once its been added to the water with that kit. There are so many meters out there and I don't know whether I need to get PPM, TDS, or EC?? I know pH is a for sure but what about the others? Please help me out! This is the only thing holding me back from starting. +REP to those that give me hand of course.

I have a COM 100 EC/TDS meter from amazon.com at $55 and a Hanna pH meter from ebay for about $30. I am very happy with both and unlike some, prefer them separate instead of in one unit.

I use a truncheon meter by Bluelab, cost me bout $100 brand new. It measures EC/CF/PPM, really simple to use. Any meter you decide to go with make sure it measures EC as the EC value is usually constant from one meter to the next, but ppm or tds can be quite different.

K thanks guys.