You're right, the bars don't creak when even someone my size gets in, but the car does seem to sway a bit more when I use the nerf bars since the nerf bars stand out from the car a few inches as compared with the door step area (longer lever arm, same force, for all you phyics junkies)! I think there's also a little bit of give by the nerf bars -- in the same way that the car sways relative to the ground, the nerf bars, for someone my size, seems to sway a little bit relative to the car.
Quick installation overview: The nuts of two fasteners of each side of the car need to be removed. The Steelcraft mounting hardware sits snug onto the fastener (the four bolts, two on each side, are fairly long) and the original nut is used to keep the mounting hardware on. I'm not sure how the Westin's work, but I'm guessing it's pretty similar.
If you have a torque wrench, I would ask the Toyota guys how tight these fasteners should be tightened as these are among the fasteners which fasten the body to the frame. Else, do as I did and make a puppy dog face to the Toyota service guys at your purchasing dealer and have 'em check the torque setting.
You won't be disappointed with the Steelcraft bars! They rock! Which color option are you going for?