It depends, but you have already some other synths, especially the JP8000 is good for in use with the nm (I use the nm togheter with the JP8080). With the nm you can emulate the lead II completely, except the arpeggiator. But I think, the Lead has such a cool keyboard, it would make enough sense to keep it. Especially the Pitchstick is sooo cool! I have the nm keyboard version and I love it, i use it beside my computer, also for sequencing mostly. It always depends how you work, but I m sure that the keyboard version is better for editing patches on the PC because you can lay your hand on the wheel and joybuttons. The rackversion is 3sided so it has not the same komfort. further you have the knobs nearer on your hands than if there is another keyboard in front of the rackunit.
In a way I would prefer to sell the prophecy because of its interface, but you cant replace the Prophecy by the nm of course (I have the Z1, therefore I sold the Prophecy).