I have a 594 Solid and 594 HBii (not a Semi-Hollow) and a Special - although mine is the Ltd edition Special with the 58/15MT's so any comparison I could give would not be quite the same.
In my case, I would say the 594 is more 'Vintage' with that more PAF like pups so the Semi/Solid is more like a 'vintage' Les Paul and the Special has a bit more bite, more a 'classic' HB and more versatility with the NF and superb split sounds.
My advice would be to try them both and see which you prefer to buy first. The neck and scale are slightly different and of course, the special has a trem vs the 594's Fixed bridge. The 594 is a PRS modern take on a vintage Les Paul - where as the Special is a modern and extremely versatile instrument and could be ALL you need to take to a Gig...
This is my 594 HBii...
and this is my Special...
Any way good luck and welcome in....