Dreambox also doesn't do much explicit instruction, at least at the early grad levels, but it seems to be based around trying to get kids to develop their "number sense." It seems to be solidly research-based and well designed (from a cognitive point of view).
They are very different stylistically. Dreambox is a whole video-game-like environment, where you go to different locations, work your way through little stories by completing math "games," earn tokens, go spend your tokens at the carnival on just-for-fun games, etc.
From the little I've seen, ixl is much more low-key and straightforward. I'm considering it as a supplement for Dreambox because Hanni gets overly wound up by Dreambox, and too focused on the rewards.
I'm liking both programs as supplements to what Hanni is getting in school (which I've been delighted to discover is an excellent research-based curriculum, Math Expressions by the Children's Math Worlds Research Project).