Alright, I'm 169cm and I went for the 16" back in '06.
At the moment I'm riding hubby's Alias which is a 17" HT and it's probably a little on the big side (in top tube length), but just slightly. My bike now is a STP in the regular size, but it has an ETT of 560mm just like the old trance had.
It probably doesn't help you much though. Just consider that if the 18" is slightly stretched out you can always go for a shorter stem, but if you find the 16" too cramped it would be harder to make it fit (if you couldn't get used to it). So really depends if you like plenty of room in the cockpit or if you like it on the small side?
Don't worry so much about the frame sizing number (ie 17", 16", 19") but rather consider closely the effective top tube length. This will give you a good idea. For example if your current bike is a 17" with a ETT of 570mm and a 100mm stem, and you're looking at a 18" Trance that has a ETT of 580mm with a 100mm stem, well you could comfortably say that the fit should be fine and you may potentially want to swap for a 90mm stem. Alternately if you went for the 16" Trance with a 560mm top tube, well that's only 1cm shorter, if it was comfortable it probably wouldn't take too long to get used to it. (above figures are theoretical).