I do not own either CPU, but I would not be surprised if i3 2120 would yield better performance than Phenom x4 as most games at this point are optimized to dual core processor.
As more and more games begin to utilize 4 cores, you'll see Phenom x4 dominating.
By the way, Phenom, as far as I know has unlocked multiplier, so you can potentially overclock it in the future were as i3 2120 is locked and what you see is what you get.
Just my 2 cents,