How much ground clearence do you lose by installing the metal sidesteps? Does it make much of a difference?

Not sure the exact amount but you can always take them off before you go off road.

3" maybe? I took off my chrome steps (I would have ruined them anyway) and put on rock sliders!

I will be taking off the steps tonight and installing the rock rails Friday night. Plan on making the mod to the mounting brackets so I can have both installed and just remove the steps when wheeling.

That's the way to go. If you are at all worried about ground clearence issues, then make the investment in a set of rockers instead of nerf bars.

You don't lose much clearance, but the real issue is that the steps aren't meant to be hit. If you hit them hard enough they can actually bend up into the rocker panel and cause damage.

Like the others said, if you plan on doing even moderate wheeling, lose the steps and get rock rails.