Opinions on Mi3 and MiUI (especially with respect to upcoming Nexus 5)

I am thinking of getting a Xiaomi Mi3. All the specs look great and so does, obviously, the price. But I recently saw the Nexus 5 and the specs are almost the same, and some reports say that it is as low as $349, which is as cheap as the Xiaomi Mi3. If that is the case, which should I get?

Also, has anybody tried Xiaomi's custom UI for Android, which they are calling MiUI? It looks a lot like iOS and I was wondering if it takes away from the interface of Android, you know, with the gadget's and stuff. (Sorry about my terminology, I don't have that much experience with Android.)

And what is the big deal with 'clean' versions of Android? Are these necessarily better, and are OEM modifications on Android like the bloatware that come with Windows? What are some of the hassles? The Verge seems to love these clean Android phones, as shown in their reviews of the HTC One when bought clean from Google.

If I get an Android phone with these OEM modifications, can I revert to the 'clean' Android? (I'm guessing not...)

Thank you in advance!