I'm just wondering about the practicality of these two gauges, and which one might be more useful.

I know having oil temp is nice so I'll know when oil temp is up to normal and it's okay to boost, but what do water temp readings tell you exactly?

 

Water temp tells you the operating temperature of the entire engine.. Oil temp really only reflects the operating temperature of the bearing surfaces and turbo... although oil temp and water temp don't really correlate, a jump in one, without a jump in the other can indicate several problems.. Raised oil temp could indicate increased friction in the motor, which may not necessarily be reflected in Water temp because most of the heat in the water temp is generated by the combustion chambers (water jacket in the head and block)
Also, an overheating engine would probably indicate other problems.. Its really just a diagnostic tool, and knowing the exact temp isnt really necessary.. Just knowing that its different than what is normal..

 

Realistically, as long as your thermostat is doing its job you will see coolant temp rise from cold to the thermostat threshold temp then it will cycle through and keep the coolant temp at a fairly static level. It may run slightly warmer in hotter temps, but overall I find that water/coolant is a lot less useful than oil temp.

Oil temp on the other hand gives you a true look at the engines operating temperature environment. As you mentioned this lets you know when the oil is up to operating temp for full throttle turbo usage, and also lets you know whne the oil has cooled down for shut down.

The combination of EGT and oil temp is an awesome set-up for monitoring a turbo car because the EGT's will rise and fall very quickly, but oil temps will have an overall affect on peak EGT's and recuperation time. Personally if I had to have just one gauge it would be a boost gauge, two gauges I would add the EGT, and third would be the oil temp followed by fuel pressure and water temp.

Again just my .02 but these are the gauges that will keep you from grenading your engine , particulalry if you are running non-stock boost and other mods.