Harsk vs Merisiel
1.) Harsk's d8+3 on ranged combat averages a 7.5 with a min of 4 and max of 11, Merisiel would average a 6.5 with a min of 1 and max of 12. As stated by Orbis, this makes Harsk a much more reliable ranger.
2.) Harsk's weapon proficiency gives immediate full use of all ranged weapons with a preference to crossbows and throwing axes, Merisiel's high dexterity, higher strength than Harsk, and no initial weapon proficiency can make daggers a more viable option and makes longbows better once she has her weapon proficiency
3.) Harsk's ability to add to combat checks at other locations puts him in a fantastic support role while Merisiel's ability to add to her own combat check when she is alone makes her a great solo choice.
4.) Harsk you can look ahead and plan out each encounter. But Merisiel, with her evade ability, can just blindly run into anything and evade whatever she doesn't like.
Bottom line: the only similarity is they would both tend to use dexterity based weapons. If your friend was treating Harsk as a crappy Merisiel than it is definitely a good thing he switched to Ezren. If you couldn't tell from this post, I really enjoy playing Harsk...