ABS is definitely superior. Check the plumbing section at the hardware store; look for the ABS glue close to the black pooptoobs (not to be mistaken with the PVC glue - those white tubes).

I'm using ordinary amazon PLA, and I was able to separate by hand the 2 pieces just now. Took an effort, but it did separate.

You do bring up an important detail about PLA; exposure to UV destroys it quickly. I'm using LED lights in the basement, but I want my original prototypes to live long without being OCD about keeping it in light-proof wrapping (I don't want to reprint later, then refinish the surface again - I know there's a lot of detailed work involved - I've done something similar before).

I guess that means development and testing with a glued-together PLA prototype (I'm gonna try Gorilla super glue - they tend to make quality products), then printing an ABS unit when I'm ready to make the mold.