Which Xeon? I'd recommend the Xeon1230v3 which is essentially the equivalent of an i7 4770, just with a little bit lower clockspeed.
For an i5 4670k you would need a Z87 board to get the benefits of it being unlocked, whereas you'll be fine with a cheaper B or H series motherboard on the Xeon. As you probably know, the Xeon has 4 cores and 8 threads compared to the i5's 4 cores 4 threads. Those extra 4 threads aren't used much in tha mjority of games - however it seems as if some developers are switching to multithreading so they can come in handy. In addition they would be useful for multitasking especially while gaming, such as recording, livestreaming or even just leaving a movie playing in the background.
Personally I'd go for the Xeon. The difference between it and an i5 4670k is negligible - the i5 can overclock to get better gaming performance. However, you'd need a Z series motherboard and a good aftermarket heatsink, and even so many only top out at around 4.2-4.5GHz, which isn't worth the extra money IMO. I'd say you'd be better off with a Xeon 1230v3.