I’m definitely not an expert… just someone who has spent a lot of time figuring out what TT size works for me. Understanding there’s a lot of unknowns here… Most bikes have some adjustability up/down and if you’re right between sizes it probably doesn’t matter as there will be decent overlap in possible setups. Think about things that would push you to one side or the other (flexibility, leg/torso length, etc). For example if you have a long torso then get the med and if a shorter torso get the small. It’s easier IMO to get the stack right, assuming you don’t need to go below the med slammed (if you think that’s a possibility in the future def get the small). Also, if you have poor flexibility maybe size up.
If you really are in between and both would support your pos, some would argue to get the larger frame as the taller frame is more aero than the things you have to do to add stack… but I don’t think it matters and would pick whatever you think looks better.
FWIW, I went with a TT bike that is considerably smaller than my road. I’m 188cm and ride a 58cm tarmac with 81cm saddle height, 120 stem, yada yada.
My TT bike is a medium argon 18 E-116 and it fits great and has some room for me to drop the front end over time. Easy these days to add some stack to a smaller frame if you need it, harder to shorten the reach if the frame is too long or drop the stack if it’s too tall… I am all legs so went with a shorter frame. I could easily get the large into same position, but the XL would be too big (long).
At the end of the day even an estimate of your initial pad X/Y as well as long term desire (i.e. maybe drop stack over time as you get used to pos) would go a long way towards making the decision.