D2 Difficulty - If you did not TP spam and slam potions like they were shots of whiskey it was actually pretty difficult if not crazy hard. This is of course solo and you didnt have the best of the best on your toon. Tho if you were geared you could 1-3 shot baal all day over and over.
D3 inferno - If you roll with more people the monsters get stronger and if they wanted to could make it to scale with your gear lvl as well. Then as you get better gear the challenge keeps getting harder not easier.