I was just wondering how much harder Challenge is than Rough.
The difference between rough and challenge is how big the raids are. At rough the raids are scaled down to 65% of what the raid would be on challenge.
So for every 2 raiders on rough, you will get 3 raiders on challenge. That is the ONLY difference between the two.
You can find the exact effects of game difficulty by going to your Rimworld folder.
Mods->Core->Defs->DifficultyDefs->Difficulties
Open that XML folder and you'll get a list of all the difficulty modifiers.