Backtracking in Legend of Grimrock

According to one of the developers the only thing that changes with difficulty is monster behaviour in combat,aka the higher the difficulty the stronger or smarter the enemies get.

Quote:

We have three difficulty settings that scale monster behavior in combat. On top of that there's old school mode which disables the auto map. We are also considering adding the permadeath option in a game update at some point but completing the game with this option would be very, very, VERY difficult I believe.

Source: Legend of Grimrock's official forum

It completely depends on the enemies you're fighting and the damage their attacks do.

If you are fighting an ogre, and they will hit you on average for 70 HP damage, and you have 30 protection, you'll only take 40 damage. If you have 50 evasion, let's say for argument's sake that it's a straight-up percentage of hits dodged, you'll take 35 HP damage on average (i.e. 1 out of every 2 70 HP hits.)

If you're only fighting spiders, and they hit ~15 HP, if you have 15+ protection you won't take any hits, but your evasion points will be worth less.

So, basically, the bigger the enemy, the more worthwhile evasion is. ... but watch out when you get hit.