I was actually in the opposite situation as you. I placed an order for my Mccarty about a week before the February GOM 594 dropped. FOR MONTHS I was second guessing my purchase since the 594 looked and sounded so cool and I was concerned that if I were to eventually own a 594 that it would put my regular Mccarty out of business.
To answer your question, after months of A-Bing my Mccarty against 594s, I can tell you this..
A regular Mccarty will be a tad bit brighter. The slightly longer scale length will give you a bit more shimmer and pop to the tone. It's pretty much the quintessential PRS tone. Very clear and musical. Now, with the addition of the new 58/15s, it sounds like a "best of" of most classic sounding guitars, if that makes sense. It also does single coil tones very well too. This is an extremely versatile instrument.
The 594 is pretty much designed to go after a vintage Les Paul tone. It is going to sound a bit warmer and chunkier. The addition of 4 knobs gives you a bit more tonal variation. You're basically buying a really good Gibson without having to buy a Gibson. The 594s also have the 'low turn' version of the 58/15s which I think sound equally as wonderful. Very clear and warm sounding.
The guitars will both feel a bit different in your hands. Me personally, I LOVE how my Mccarty feels. The one piece bridge just fits perfectly under my hand. But, the variation of the two piece is really nice too. The 594, to me, feels a bit more compact which feels very nice. The neck is chunkier too, in a good way.
I don't own a 594, yet, but I recently picked up a Stripped 58 Singlecut and it definitely gets into 594 territory. I gig with my Mccarty 90% of the time but some gigs I'll take my Singlecut out with me if I need that Gibson tone.
So to answer your question, I'd say get the Mccarty! I find that having two slightly different guitars can be very fun and enjoyable and not redundant at all. Some days you'll favor one guitar over the other so it will be nice to have options to choose from.