I wanted to play on one of the Impressive Title servers because I wanted to better understand FeralHeart's roots, and to understand what people were talking about when they said that they missed IT.
I eventually boiled down my options to Impressive World and Sanctum of Eventide, and chose IW in the end because it offered more species options. I didn't have to be just a lion or a tiger. I could also be a griffin or a hyena if I wanted to.
I just started playing IW and it's a little early for me to judge it yet, but I'm enjoying it so far (especially item collecting and exploring). It seems to lean more towards the fantasy element than FH does (FH seems more like a semi-realistic game setting).