I'm no fan of boy bands and back away from them like they have the plague! But to me District 3 have a lot more to offer then Union J. I don't know what it is, but where I feel the need to hurl abuse at UJ (which I do feel bad about as sure they're decent lads!) I don't feel the same about D3.
I think it's quite interesting to see that D3 don't necessarily go for the safe option, but UJ always do. Why would Loius encourage one to take the risk and not the other? To me he has UJ earmarked as THE boy band of the show, so risking D3 is just one of those things. Also the criticism they aim at D3 could also apply to UJ, but it doesn't really happen. So I don't think it's just Louis who has them earmarked.