Yo,
To make something VERY clear right away, both Cubase and Cakewalk are sequencers. SoundDiver is an editor for synths.
I assume that you're interested in getting the JV1010...in order for you to edit any of the sounds, you will need something like Sound Diver to do this, because the 1010 doesn't have any front panel knobs (except for the little one).
If you want to compose music, you will need a sequencer, like Cubase or Cakewalk. This sends note and music information to the 1010 and makes it play the music you want.
In an ideal situation, you would run Sound diver and a sequencer side by side on your computer so that you can create music and edit your sounds at the same time.
If you wish, you can make Mixermaps (in cubase) or a StudioWare (in Cakewalk) which will do the same thing as Sound Diver, but I wouldn't recommend this, as it would take you a LONG time to write the proper code to make it work properly.
I hope this helps.
Email me at leodaley@worldnet.att.net if you have any more questions.
Nick