I haven't read the Spice and Wolf LN yet (but I plan to), but after having watched both the old and new anime adaptions I wanted to write a little bit of thoughts about them.
I think preferance of one over the other is ultimately a matter of taste. I think the newer version is a more faithful adaption because it doesn't skip over the Tereo arc. At the same time I liked the soundtrack in the older one better than in the newer one (the newer one has good OST too, but the older one is just PEAK).
I can't really say one is better than the other but if I was forced to compare them, I would say what I wrote above