Its pretty obvious why they didn't sing Cannonball - that song was totally unsuitable for them and its pretty crap in a boring way as well. Don't Let Go is a pop classic and they probably haven't had any songs of their own written for them yet. This is the best I've heard them sound so far - on the X-Factor their harmonising was more than a little dubious, frankly I thought they were overrated on the show. However, they certainly nailed that performance so I'll reserve judgement until their own material is released ;o)