Stock for stock, the i will handle better and do most of the performance aspects better besides launches.
Install a tune and I feel the speed benefit goes more to the xi. With more power, the xi can handle it much better vs the i being rwd. Unless you're going over 35mph then the extra weight and drag of the xi hurts.
Now move to handling, and the i with sport suspension will kill the xi stock. Now toss some good coilovers on both the i and xi and there will be a much bigger benefit for the xi since it's initial settings are so soft. Being I've never driven a lowered i, I can't say which one is better after coils.
Overall, if you don't plan on modding and want to stay stock and don't need awd, I'd say get the i. But if you plan on modding all out like I am, I think the xi might be the better platform. All the i guys will give me sh%t for saying that though.