hold vs toggle, what do you prefer?

Always hold for ads. Toggle is extremely clunky and you're also prone to do more mistakes using it, especially under stress.

As for crouch/slide, I have a weird setup: I have hold slide on Q, hold crouch/slide on Ctrl and toggle crouch on C. I use them all.

I use Q for just sliding when running - it's just a more convenient button than Ctrl for common use. I use Ctrl for some other maneuvers (where Q is inconvenient, e.g., backward slide mid-combat) and I use C when I need to crouch for extended periods (e.g., healing behind a small rock).