I asked the same question at the recent Magic Live in Philly. One difference is that the Justice Pad is larger. Another is that it's easy to get pen replacements for Psypher. (That may change for the Justice Pad when it has been around a bit longer.) My seatmate who had a Justice Pad with him was impressed enough with Psypher that he bought one at the event. He said he plans to use Psypher for cards, and continue to use Justice for the straight mindreading/picture replication he does currently.