Which of the two gauges seems to be the most accurate? The digital gauge fluctuates quite a bit but the manual gauge stays pretty consistent. Thanks.

 

Digital gauge in the DIC is more accurate, the other dash board temp gauge is more of a decorative thing!

How old is your Dexcool? Your thermostat? If you just replaced the thermostat is it an AC Delco 131-158 180F ?
Did you burp the coolant system?