The full zip and half zip shaks are exactly the same material. If one is softer or more stretched out it's just because it's older or has been washed differently, etc. Another possibility is that you wear the full zip OVER more other stuff, so it stretches more than the half zip which you might wear by itself or over only a thin base layer.

And yes I prefer my full zip shaks a bit more because they are so versatile. Wear them alone like a pullover, or wear them over stuff like a cardigan.