Hi Thim! Can't find OS comparison here but there are threads like this for MiUI: Give Us Your Feedback for MIUI 9.2 Global Stable ROM! https://c.mi.com/thread-739933-1-0.html
I highly suggest for you to watch YouTube on the software review of both. It's best for u to see how it really works plus the features instead of reading. It's your phone to be anyway so we better make sure you will love the OS in your own preference. Threads here do open some of the bugs or issues so please consider that too before buying your phone.
Should you have questions we're here to help!
So far I am belly satisfied with MIUI software until to date, will give opinion if I have a chance to experience the Android (wasn't really into it last time with my HTC One. Honestly it was a good phone though)