You want your sight glass to start near the bottom of the pot. If you have a combo fitting, then this will be where your thermometer will be. This is beneficial if you are running something like an HLT and the liquid in the tank is more or less going to be almost the same temp. You can also measure the temp with a low level of liquid in the container.
In an MLT, I could see how you might want the thermometer somewhere higher than the bottom, but this is coming from someone who doesn't have a thermometer fixed to his MLT anyway.
For HLT, my vote is to go for the combo. For MLT, maybe combo, maybe not, depending on your personal preference of thermometer location, if at all. For boil kettle, I can see a benefit of having a sight glass, but not really a thermometer. I just use a dip thermometer in my MLT and BK to verify temps.