I would recommend the step-in (first shown). We have something similar to the second one, but the neck hole is not adjustable, and it is always a little bit of a struggle to take it on and off.
Puppia just came out with a fully adjustable one, but unfortunately, the smallest size is still way too big for Thor.
Make sure you find the measurements of the harness to ensure that it will fit your dog's neck and chest if she is small. I want to love Puppia, but their "fit model" is not built like Thor.
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ETA: The Puppia harnesses do not seem to cause matts or hair breakage.