Bike selection for once in a life time A race(s)

I’m following plan builder and sorting nutrition ready for my A race in May. This consists of two races.

  1. 2.4km Time trial. no not a typo it is short and flat and no corners.

  2. 40 minute Crit.

Crit first as I think the TT will cause more debate.

I am lucky enough to have a choice of bikes, both almost identical fit both will have same wheels.

A Specialised Allez sprint disc upgraded to DI2
A SWorks Tarmac Disk DI2.

I think Id be more aggressive on the sprint but with this being a flat round (no proper corners) course Im not sure I would get more advantage from the lighter Tarmac.

Next the TT. Yes 2.4km flat no corners as far as intel suggests.

What do you think will be the net gain of a Time trial bike over this short distance. Bike fit and position dialled in over a road bike with Aerobars fitted? Clothing, helmet etc all must stay as supplied.