If you are between sizes, I'd suggest the smaller of the two sizes for TT frames.
But as everyone else is saying a sizing by a good fitter is a worthwhile initial investment prior to purchasing.
I ride 56-57cm road bikes (183cm tall - 86cm inseam) and have recently bought a 52cm Alloy P3 which I am currently building up. I was sceptical about going that small, as the general rule of thumb is to get a top tube ~2cm down from your road bike. But I went and borrowed a friends 52 cm P3 (who is identical in fit) and went out on a 60km training ride on the bike. Note though I am still pretty flexible in the back and hamstrings so can ride more aggressively...
The smaller frame and forward position is also fine for me for Tri's and TT's. As I don't race under UCI rules, so am not restricted by UCI rulings in regard to seat position behind the BB. Which would be near impossible to meet with the extra small frame and forward seat position.