Memrise is fun, and certainly much better designed than Anki, but at the end of the day it's just another startup that will either fail or get acquired and shut down. All your content will be destroyed, or maybe, if they're generous, given to you in some crappy export format that won't be interoperable with anything else.

I use Memrise, but Anki is my extended memory. Anything I want to actually remember is there.