I have both ergo and bj, from my experience, I think bj is suitable for small size baby while ergo is more suitable when baby has grown bigger in size.
This is cos according to ergo's design, baby's legs have to be widely separated when using it, my baby (around 4mths old) cried everytime after I put him on after 5mins until I took him off...
Then I tried bj, the problem gone, in bj's design, their "浪" is not so wide, so baby's legs do not have to be widely aparted.
But I must say, ergo's design is more suitable for heavier babies, doing less harm to mummies' back.