As to which option is better, it all depends on what you are wanting to do. If you are only using a few instruments, and/or writing primarily chamber music, or small orchestral scores, you probably can get by with adding an SSD to your existing computer (you don't give the specs, so one can't be certain). If you are looking to do large orchestral works, with many libraries, in MIR Pro, then there would be a real advantage to getting a slave computer to run the VSL libraries and MIR Pro, while keeping your existing computer for your sequencer, and whatever other programs you might be using.
As for the size of the SSD, it all depends on how many hard drives you can add. Adding more than one can be helpful as it spreads the load out. Note that DAW computer builders have generally commented that there is no real advantage to using RAID for streaming samples (they also say that if you are doing heavy video work, then RAID does have some advantages even with SSDs). IMO, the 240/256 size of SSD is good, though as prices continue to drop, that could change.