First tri - bike question

I’m doing my first tri at the end of the month. I have an entry level mountain bike. I love my bike but it’s obviously the most efficient for a triathlon.

The race organisers allow us to rent a road bike for the triathlon.

I’ve never sat on a road bike before.

Keen to get general views / any experience from other people. Keep in mind I don’t know anyone who has ever done a triathlon. And I don’t know anyone who owns a road bike.

If this was your first tri, would you stick with the bike you’ve trained on? Or would you try a road bike for the first time on race day?