It really depends on where you are based and who you fly more often.
If you are based in the America and you tend to fly to Europe, I don't think JMB is a better program for you than AA. Within the America JMB may have some advantages given that you still get lounge access rather than needing to pay for Admiral Club, but this gets diminished if you have a high annual fee card. However, if you are flying to Asia, then I think JMB has the edge since JAL may become your preferred carrier..
Also, you could just try JMB for a year and then if you don't like it switch back to AA, and then retain your JGC Sapphire.