They both are the same... buying gum is buying gum.
In Classic... killing one mob within one level of yourself... easy. Kill two? Depends on the mobs... you might be in trouble. 3? You are probably going to die unless you really are on top of you limited skills. In modern WoW? You can't buy a challenging engagement.
Yes of course encounters are more complex in endgame, but are dumbed down with dungeon journals an big giant arrows telling you to run here, and not stand there.. Your characters are also more dumbed down with pruned skills. You rarely have to CC, spellsteal, counterspell, worry about threat, about mana, about bandages.
So are modern WoW encounters "harder"? I suppose an argument could be made for that but I'd argue that they are different... and therefore you cannot compare difficulty.
In the end, all that really matters is: Is it enjoyable?
I, for one, would much rather run vanilla dungeons than modern ones. Why? Because of the journey it requires, the preparation, the class fantasy of having to sheep a mob, or de-enrage one... to use fear ward because I know the boss fears.
To want a boss dead because he drops an item that will cap my hit, my defense, my block.
Not a dungeon I can simply gear doing a few world quests and jump into with 4 strangers via a queuing system, easily clearing it the first time because I read the in game journal, only to get another boring piece of gear that has basic stats no better than te world ques gear I am already in.
Wipe in a modern dungeon? Please. Death is meaningless anymore. Corpse run a Deadmines lately? Yeah you don't wanna do that again.
I mean, if it makes you feel better to think modern encounters are "harder"... by all means go ahead. I will enjoy Classic and it's "easy dungeons" for the next couple years... which is better than the 3 months the past few expansions have been able to engage my attention.