Well, our boys have specific tasks set within Mathletics by their school teachers each week, so its far more specific to their curriculum and whatever they are learning.
They both enjoy using it, but prefer the credit system you can build up to earn games and buy things for your virtual bedroom in Maths Whizz, so Maths Whizz seems to be a bit more motivational for them.
For me, what I like about Maths Whizz is that if a child doesn't succeed at a topic, they keep working it back in, dropping back the complexity to a point where they can do it and then working up from there. They keep getting the same topics back until they pass the assessment, but not in an incessant, going over and over until you've got it right kind of way.
When I was first looking at online maths programmes, I chose Maths Whizz over Mathletics because I felt with Mathletics there was too much emphasis on competition and league boards, which I felt would be demotivational, especially for my ds1. I liked the way Maths Whizz rewards individual success and encourages children to go back and try the exercises they've already completed again and again to better their score and earn extra credits. I felt that was rewarding individual effort, rather than setting child against child, iyswim.
This seems to have been borne out to some extent since the boys started doing Mathletics at school, as there's lots of emphasis placed on being top of your league and getting the school to be placed top in the school leagues as well, rather than focussing on the learning and rewarding that.
You can do challenges in Maths Whizz, where you complete a timed exercise in competition with another child of the same standard, but you don't have to and my two don't tend to bother.
I was also annoyed that I couldn't get a proper look at the Mathletics exercises without signing up, so wasn't sure how 'gamey' they would be, whereas with Whizz I knew that the boys would get to learn in a fun way, but then have a straight maths/numbers assessment to make sure they'd really learned the topic. Since going on Mathletics, both boys have said its not as much fun as Whizz.
Hope that's helpful. 