I think Little Mix were more likeable and had great chemistry with their personalities and seemed more like an established group on stage. We don't really know much about any of the boys except that there are girls outside screaming for Union J every week (been their VT for 2 weeks) and District3 are trying to dance (again VT for 2 weeks). Little Mix's personalities were shown through their VTs.
Also there was more of a gap for a girl band whereas there's already One Direction, the Wanted, Lawson, JLS, I think a lot of people may be bored of boybands.
I was surprised Union J was in the bottom 2 though, was expecting District3 as I think they seem less likeable.