I let the game decide at first. It tells me that I will at least experience most of the stuff the devs wanted me to experience in their game so I don't miss a single thing.
Then, the harder difficulties are just a very nice bonus. Nowadays, with achievements and trophies it's even more worth it to play these difficulties. Sure, bragging rights are still important...! 
If I just want to have fun and not worry about dying on bastard enemies, I'll just play the difficulty that still gives me all the game's features, but with dumbed down enemy health or death traps, etc.
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...
