I'm not an expert on either of these parks, but I do know a little bit about Phantasialand, so I've put together a small list of reasons why Phantasialand is the park I'd choose to visit.
-World class theming, very immersive
-Two brilliant onsite themed hotels
-Great selection of rides for different age categories
-Closer to the UK, so likely cheaper to get to
-Day ticket is €37 or at today's exchange rate, £33 - which for what you get, it is relatively cheap
-Relatively small, so easy to walk around
-Fantissima - which I don't know much about, but it sounds interesting to say the least
Hope I've been of some help!