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Hi there, I am not (yet!) an expert in this area, but I'm working on it and know
enough to give you a basic answer to the above question.
The DAW/Workstation combination would consist of your DAW plus a keyboard containing
your sound banks (sometimes referred to as voices).
In the other combination your sound bank is contained within the rack unit, then you
would have a seperate keyboard with which to 'control' it.
The reason this can be handy is that these MIDI controller keyboards are designed
specifically with this purpose in mind, and usually have a greater number of hands-on
controls such as knobs, sliders and buttons with which to manipulate the sounds/effects etc.
So you end up with three seperate units which each have a specific function.
This is absolutely ideal for producing home-studio music, but not so good for performing.
The difference with a workstation is that it is designed to do 'everything'...
so from playing the original idea, to ending up with a finished recording - obviously, in your case,
with the assistance of the DAW - it's pretty much 'self-contained'.
The main difference you will notice is that the actual 'feel' of the keyboard is
more user friendly, from a playing/performance point of view, on the workstation
compared to the controller keyboard - these tend to be very 'plasticy' & flimsy in feel
(although they are getting better).
You would certainly notice the difference in comparison to your FP-3.
If, however, you don't intend to perform live using this set-up you may well be
best off with the rack/controller option.
To illustrate the point I'm (badly I think!) trying to make have a look at the Korg Radias below - the keyboard is basically a seperate unit from the 'rack', which contains all the good stuff!