First, go to Competitive Cyclist fit calculator and do a little fit home work before you buy.
competitivecyclist.comCCYOpen ↗
I had a TCR (2008), I'm about the same height, and have a 73.6cm inseam. I rode a small and Giant use to post an approximate height range by bike size on the down tube. The small cut off was 5' 7". I'm a little more concerned by the top tube rather than frame size and generally favor 53 to 53.5cm TT, which in a lot of bikes is a 52cm frame.
If you are going to put clip on aero bars, I say stay with the small as the medium will stretch you out and not allow for a good tucked position. Of course this all depends on how long your torso and arms are. Depending on your flexibility, I would also keep an eye on how much room they left for you to adjust the stem up or down. There's nothing more silly than a racy looking bike with a gooseneck stem point straight up in the sky to get your handle bars to proper align with your seat height.
Lastly, I would ask around about how well that shop fits bikes. You’re really asking for two different fits for one bike.
As a side note, I didn't like my TCR. It had kind of dead ride and I always felt like I was dragging the backend up a hill. I actually, would look behind just to make sure something wasn't wrong, low tire or brakes touching. Same wheels and drive train with a different geometry and that sensation went away. That's just my experience, I know folks who love their TCR. It doesn't hurt that it's a great looking bike.
Buy fit and not brand, good luck, and have a fun tri season.
