The biggest thing to look at when sizing is the top-tube length. The NRS and Trance are 2 different styles of bike. The NRS is more "racer" hence the longer top tube. The Trance is a more "trail" bike which could be used for racing if needed. The T.T. length on the Trance is just a bit shorter than the NRS, so it will give a bit of a more upright and comfortable ride. I ride a 18.5 NRS Air, and if I went to a Trance I would head straight to the 18. You should be able to see the difference in the animation I made up below.
I was riding a 2003 NRS size 18.5 and recently swapped the parts over to a Trance 4 (black) frame size 18. On my test ride the fit was fine. Got home and swapped in my thomson seatpost, only to discover that the cockpit felt cramped. The stock easton seatpost allowed the seat to go further back that the thomson post did. I had to order a thomson layback post and that cured the problem! Trance has a more upright feel to it over the NRS. I suppose I could get a longer stem and go from 90mm to 100mm? I did read that others riding an 18.5 NRS went with a 20" Trance. Hopefully I did not choose the wrong size.
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