It depends on what you want to play. If you want the most immersive experience possible, the Rift is the way to go. However, it only truly shines on games which were designed with virtual reality in mind. It's really not very good for traditional games designed to be seen on a flat screen.
There are lots of reasons why.
1. The Rift is meant to display 3D. Games without careful 3D implementation are often ugly and can be unplayable in the Rift. Menus may be unusable in game. The 3D effect may be done so poorly that it looks less realistic than without 3D.
2. The Rift is meant to use head tracking and most current games don't support it at all. Very, very few games give you the ability to look around with your head separate from moving your body. You can usually get the Rift to emulate mouselook, but when moving your real life head to look around causes your player character to move their whole body, it's disorienting.
3. Player character movement in the Rift can be nauseating, especially on games with quick movement. Player walking and run speed in most games is ridiculously fast compared to your speed in real life and it feels bizarre moving around in games like that in the Rift.
4. The Dev Kit is not a consumer product. The build quality isn't bad, but it's not rugged or reliable enough at this point. The resolution is crazy-low and the screen door effect is very noticeable. Word on the street is that a second dev kit will be released soon with at least some enhancements.
I don't want to sound like I'm bashing the Rift here. I absolutely love it. But, I mostly only use it on games designed for it. The only two titles I've found enjoyable that weren't designed for the Rift originally are Mirror's Edge and Half Life 2 VR.
On the other hand, multi-monitor configurations are not as immersive as the Rift, but they mostly work very well with existing games.
Why not both?
*Edit: I see in one of your other posts that you're specifically interested in racing sims. iRacing does support the Rift, but I don't know about Project Cars. I can say that I prefer using three screens in racing sims versus the current dev kit Rift. The resolution is just so low on the current model that it's too hard to see track markers and other important details coming up on the horizon. My uncle is a more serious sim racer who was interested in going triple-wide or Rift. After trying the Rift, he decided to go with three screens instead.
What I'd really recommend is going with three screens plus a TrackIR or other head tracker for racing.