Well, as rightly said above, it boils down to the personality of the player and the "shadowling influence" can be explained for both cases- after all, one can go through the game with the Main not being significantly affected by the Shadowling customs (by the right selection of replies). Plus, there are humans on his team as well, and Grotius too, so they can give him a slightly different perepsctive on certain issues (and let's not forget that to of the Main's best friends are on the team as well, so they'll probably spend a lot of time together and influence the Main).
Personally, I keep them both alive, as not only do I use Grotius and don't need the personality dip, at that point in the game I think the Main has enough info on his hands to be a bit more cautious of what the Shadowlings tell him and what Nelis stands for in the Shadowling society (cf Anya reading local books and telling that info to the Main).