I have an eyefinity setup and it is great and definitely worth the money.
Unfortunately I can't run it at a high enough fps because the AMD crossfire driver isn't working properly yet (flickering reflections and textures). Still, I've played with it a little bit in TW3 and it really immerses you in the game and it's environment. It works even better in first person games, GTAV is a great game for it.
After playing with eyefinity for a while you don't want to go back.
I can't really comment on 1440p since I have only had 1080p displays, but as that is simply a 16:9 or 16:10 screen with a higher resolution it would be less immersive then eyefinity. You will have less trouble with aliasing though. It probably also is less demanding because there are fewer pixels.
Keep in mind that if you want to build an eyefinity setup and you manage to get yourself a couple displays for cheap, the hardware to run the latest games with it will not be cheap. I need two AMD R9 290s for The Witcher 3, and unfortunately AMD isn't really quick with their crossfire support so I'm currently stuck with playing on a single screen.
An extra benefit with eyefinity is that you have two extra screens to divide your work on, so it's easier to do some multitasking
Two, I don't enjoy moving my head left and right while gaming.
It may seem silly at first but you aren't supposed to move your head to the side much. It's all about your peripheral vision. That is why near the edges of the side screens you will have stretched details, to stimulate your vision in that region.
A great example of this is driving a car in GTAV in first person, you will see the pedestrians and cars fly past you like you would in reality.