Be sure that whatever you get has two point calibration. Also be sure to get good quality calibration fluids.
I discuss what to look for in a pH meter (and how to use one) here:
Measuring pH with a Meter
http://www.advancedaquarist.com/issues/feb2004/chem.htm
from it on how to select a pH electrode:
"Selection of Combination pH Electrodes
There are hundreds of different types and styles of pH electrodes. These vary in size, composition (glass or plastic sides, typically), shape, the nature of the glass membrane (even non-glass materials can be used), how that membrane is separated from the internal filling solution, (that is, single vs. double junction), whether the internal fluids are sealed or refillable, and a variety of other issues.
For most reef aquarists, the exact nature of the pH electrode doesn't much matter. Nearly all can give suitable pH readings. Here is my opinion about what is most important to look for:
Selecting an epoxy body electrode instead of a glass body electrode will make it less likely that you will break it by accident. The glass bulb on the tip is still very fragile (I've broken many), but the bulb can be protected by a plastic shield to reduce the chance of breakage.
If you intend on measuring pH directly in the aquarium or sump (rather than in a cup of water removed from the aquarium), then expect to occasionally get the whole electrode wet (by accident). If it is a refillable type which has a hole for adding more filling solution (usually KCl), then you might get aquarium water into the electrode. That can severely degrade performance by messing up the reference electrode. So selecting a sealed electrode may be a better choice.
Some folks argue that a double junction electrode is preferable, and that is likely true. Without going into the design details, a double junction electrode makes it much less likely that there will be any exchange of ions between the aquarium and the filling solution through the glass bulb. This option may be especially important if you are using an internal reference electrode other than Ag/AgCl. For example, if it is a calomel reference electrode, it contains mercury. It is desirable to keep mercury salts out of the aquarium due to its toxicity, so a double junction is likely preferable in that case.
Likewise, it is desirable to keep sodium and other seawater ions from entering the reference electrode, and a double junction prevents that to a much greater extent than a single junction. That aspect is more important if you leave the electrode in the aquarium water than if you put in into a storage solution after every use.
In general, it is likely better to have a double junction than a single junction, but it may not result in appreciably better measurements.
There are other attributes of certain electrodes that may make them more useful in laboratories, and hence justify prices that easily range to more than $350 for the electrode alone. There is not likely any significant advantage in using such electrodes for reef aquaria, however. Do not be worried about claims that you need a special electrode for high pH or high sodium environments. That is true for the most accurate measurements at high pH and high salinity, but aquarium water will be OK with a standard electrode, and even brief measurements at high pH (like limewater) will be fine.
An electrode that I have been using for the past few years is sold by Cole Parmer. It is catalog number U-59001-70. It has an epoxy body, is sealed, and has a double junction. I'm not certain what type of reference electrode it contains, but I believe it to be Ag/AgCl. Its list price is $60."