Haven't noticed much difference. Both PT and 18K gold Langes will show signs of wear pretty soon, no matter how careful you are] As far as dings go, I'd venture to say that pretty much any impact strong enough to cause a ding in a gold watch will also cause some sort of ding in a PT watch. I'd also venture to say that most folks who are bothered by dings will be bothered just as much by a small ding as a large one. So in short, personally I wouldn't give much weight (no pun intended) to scratch/ding resistance as a factor when deciding between PT vs. gold.
The good news is that you can always hit either a PT or gold Lange with a polishing cloth to give it a quick shine, and Lange can return it to you looking like new whenever the watch gets serviced (or more frequently, if you want).
Don't know the PT alloy that Lange uses (iridium, ruthenium, etc.), sorry.