This was someone's response to a steam forum thread about Civ 5 vs civ 4.
Civ 4 was... complicated, to say the least and anything beyond prince difficulty required levels of micromanagement the average gamer could only dream of, rendering gameplay a lesson in mathematics. Combat pretty much boiled down to the stack o'doom.
Now that's not to say I didn't love Civ 4. It was awesome.
But Civ 5 is slightly more my style. Yes ok, partly because it's easier, but for all the best reasons. With no religions you're no longer automatically the target of hatred, indeed it's finally possible to play peacefully and manage not to get goomba stomped into the ground by Alexander the moment he shows up. The stack of Doom is gone as only one unit can occupy a tile, making warfare a little more strategic. No more stacks of a hundred catapults to soften up a city. And most difficulties cities can generally micromanage themselves pretty well, although the purists would obviously be able to do it better. And resources are more strategic, having a source of iron no longer equals 3000 swordsmen, making acquiring these resources far more important. Having a swordsman when your enemy does not is now game changing.
Personally, I don't think the game has been dumbed down so much as retargetted at those gamers who don't have a PHD in statistical mathematics. I know some hardcore civ 4 players that would find it way too easy even on deity and hate it, but people i know who found civ 4 way too difficult on settler find civ 5 far more rewarding.
To answer which is better, I guess it comes down to how much do you want from it? If you want the challenge of tweaking every little thing always one step away from economic collapse or all out world war then civ 4 is your man. If you want something a little easier to slip into that still feels like Civ then 5 is what you want.