From my experience I'd estimate 80-20 in favor of the rails on the bottom side of the plastic sill cover cracking before the white mounting clips would.
What's worked best for me is to get either end of the rail free of its mounting clip, then slide the rail off. A set of nipper pliers works great in grasping any clips still stuck in the sill.
If you're near a salvage yard, it's a great place to practice and pick up the spare parts you need.
Adios for now,
RM