BTW. Unless it is an UPHILL only TT that doesn't loop back, you will be faster on your TT bike unless you are uncomfortable on your TT bike -- if you aren't comfortable enough on your TT bike to suffer through the event, then put the TT bike in your trainer & spend more HOURS in the saddle in the TT position until you are.
For the time being, basically, YOU have to weigh the aerodynamic DISADVANTAGE of the ROAD bike compared with your aerodynamic ADVANTAGE on the TT bike ... so, that is why I suggest you ride the course if you have the opportunity to do so.
Choose the wheels-and-tires according to weather condition. How do you know it will be rainy?