It's a toss up for me. I'd lean towards keeping the DD, but that's because I've tried both the mpx and mcx, and they're...alright. They're good, solid guns, but they feel like any other decent AR. Your mileage may vary. If I was in your shoes, I'd keep what I have since it seems that you already took the time to properly set it up.
That said, you only really need one good AR as your "go-to." It seems like you enjoy pccs, so if you really like the sig, try it. It's really up to you to determine if the cool factor is worth it.
When it comes to ARs, your PWS is more "special" than your DD. Any good DI rifle will replace it. BCM, SOLGW, you name it. DD isn't special by any means. You can simply try the pcc and see if you like it.
I completely understand why you'd want more than one AR, but not all guns have to be 100 percent practical. I'd say the choice is even between. You can't go wrong no matter which gun you decide to keep.