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just my experience from playing with 2 groups:
Normal difficulty seems to be much to easy as my players are trying to max out their chars/combinations.
But one difficulty higher was quite okay!
You may think: It is just one hit more(and some modifications on a table), but it really made a difference. It was for the GM and the players much more fun.
I think it depends on the group really.
If you are playing min-maxers, you might want to up the difficulty from the start. If you are playing people who want more of a story feel, you might want to leave it as-is to start with.
If people are introducing the game to new players, I suggest starting on normal difficulty. If they find it too easy, you can ramp the difficulty; but it can be disheartening for a group if you smash them to bits from the start.
I find that once the heroes have picked up a grievous wound or two, the tide quickly starts to turn. Even on normal mode, I seem to be able to smack the heroes about quite happily.