Welcome to the board.
Either The Juki 2010 or the Brother PQ1500s would be great for what you want. I have the Brother PQ1500s and haven't missed the speed control- but, I learned to Free Motion Quilt back in the Upper Neolithic before there were speed controllers on domestic machines, so I am used to controlling speed using the foot pedal alone. If you've never FMQ then it might be easier to learn with a machine like the Juki where you can limit the speed with a slider so that you can concentrate on what your hands are doing. Don't know if that would make it worth the extra money over the cost of the Brother. Both the Juki and the Brother have the same threader and it is definitely more finicky than the threaders that Brother puts on their other domestic machines- so if you would prefer the speed controller, then don't let the fiddlyness of the Juki threader put you off.
Have you tried FMQ on your CS6000i? I know of several folks who have done beautiful free motion quilting and free motion embroidery on them. You might want to try some FMQ on that to get a feel for whether or not the speed control is something you want or could do with out. You can get a feel for FMQ by buying some of those cheap packages of felt from your local craft shop (Michaels or JCMoore): Layer two sheets together (no batting needed) and pin in a couple of places, then try doing some basic loops, curves, lines, etc. to get a feel for machine speed and hand speed.
Rob