I think the third one is the best of the three.
The first one's facial expression isn't the best and his head merging with the singer sort of doesn't work, the guitar is outside of the DOF and those last two points frame the guitarist's face, which doesn't look good.
Second one, again facial expression is the key and I don't think its got it. Also a slightly deeper DOF would have improved things, to include the singer. I don't like the framing either, too much space above the heads... the shot feels cramped but not tight and the space just makes me think of the wasted space.
Third one, now you are starting to get the guitarist and the guitar inside the DOF. The avant-garde angle/composition works well and his expression is much nicer too.
When you are shooting people doing stuff the facial expressions yielded really are the key to a good shot. Frame something up, track it and bang out a good few shots because you cannot control what they are doing. Thats not hosing it in my book, its just trying to get a flattering photo of someone who is not posing for a photo!
The big technical problem I think you have run into here is having to use the 50 1.8 to make up for 40D poor low light performance. My opinion is that 50mm is not a good size for a crop body and the subject distances you tend to naturally, subconciously end up with make for poor framing. I also don't like the very technical sharpness the 50 1.8 (sometimes LOL) yields, its all very inhuman and cold.
The answer is probably a D700/D3 and a Nikon 24-70 or 70-200.... superb low light AF performace, high ISO available to allow some deeper DOF, better renditions from the lenses and zoom lenses to allow for more flexible framing... unless you want to shoot totally wide and crop to compose - I prefer to use the zoom to start with. Primes are all well and good when you are doing studio stuff, less good for whats essentially candid action shots.
