The biggest differences seem to be the Christadelphians do not believe in the preexistence of Jesus, nor do they believe in a literal Satan.
Of course, really the biggest core belief of JWs is this: In 1919, Jesus chose the Watchtower leaders as the one and only channel through which God communicates his will--salvation through the impending Armageddon depends on obeying this channel now and in the future.
Every other JW belief is window dressing.