I would focus on recommending Nehrim, as it is fairly well finished and has a good Wiki, which many players rely upon. I think anyone who really enjoyed Nehrim would discover Arktwend. However, I think many who first played Arktwend might become a bit discouraged as there is no Wiki (at least in English) and quite a lot of it seems unfinished, empty rooms and dead ends, and places that look like they were going to be a quest but never were done. Don't get me wrong, I've played through Arktwend 4 times (in English), but I think it is more appealing to those who enjoyed Nehrim and want more of that experience.