Both of them are equally better.
Don't underestimate MIUI. It is Android underneath but 10x more Vivid, Stylish and Exotic as compared to Boring UI of Nexus 4. However, the MI2's Build may not appeal you as much as Nexus 4's.
At this point, i would happily purchase a used Nexus 4 in about 35k. Not the Mi2. If i were to buy the latter, i would prefer to wait for Mi3 which would be even more epic for price just a tad higher than MI2. (Rumors are that it'll be around $350)
Conclusion: Get a Nexus 4 16GB Used for as low as possible.