I haven't seen many variations of Sweeny Todd but I know that there was a BBC UK version they did which seemed dark and what appears to be more like the broadway show. But seeing as I only have Tim Burtons to go by I will comment about that.
Burtons version just focused on what i saw in the extra's on the relationship between Todd and Mrs Lovett. Thus missing out some of the original songs from the Broadway show, and looking at other characters. I even thought the killing scenes seem a little more staged and out of place. They hardly seemed natural but thats because much like my boyfriend, its better to see a more well acted death that looks realistic. Even with a more blood coloured blood would have been better.
With regards to the singing, I am sorry but Bohen Carter was a bad choice. Her voice really was week and it changing from her being in her range and being out which lost all changes of anyone really relating to a character in my opinion. Depp was able to surprise me with his singing voice. His songs where in his range which helped maintain the character through out the songs and the acting.