OK Sadie Kate....
I measured and compared my two medium zip turtlenecks today:
One is the Zephyr zipT and the other is the SmartWool zip-T, both size medium, both longsleeve turtlenecks and both have been worn quite a few times and washed about twice in warm (not hot) water with Woolite and dried very slightly on LOW and then laid out to dry the rest of the way.

I found that the Zephyr is about 1/2 inch longer in the torso (from shoulder to bottom), and about 1/2" longer in the sleeve (lined up the cross seam under the arms.)

The SW zip-T is MUCH more form fitting though- has seams that taper the waist in and the sleeves are narrower too- easily fits smoothly under things that would make the zephyr bunch up underneath. The Zephyr is nice when you wear it on top because it's a more relaxed feel.

Hope this helps.