I've been researching the same thing and asked this question last week.....
There's a lot of information on Disboards and others about MIA hotels......a lot of reading....
So far, this is what I've gleaned.....
Near Airport - good reviews...Blue Lagoon area - Hampton Inn or Homewood Suites. However - I just read yesterday a Cambria Suites is opening up in that area and rooms are really cheap - around $100 for the sat night prior and less if you are willing to prepay. It's a new hotel opening next month so no reviews and likely why the rEwes are so amazing.... Generally, Cambria as a chain tends to fare well as far as reviews go. Cab fares to port the next morning from this area appear to be decent - I read around $25 but haven't confirmed....maybe someone else can...Across from Hampton is a little strip mall with a few dinning options and sounds walkable from other 2 hotels.
Downtown - I'm leaning this direction to have things to do the day/evening before and better dinning options within walking distance. Hampton Inn Brickell gets really good reviews and isn't much more than the above named Blue Lagoon hotels. I have found the price keeps going up and down - so I keep checking it.
I know you said MIA - but just in case...we are flying into FLL and considering staying there and taking a shuttle to port on Sunday morning. Shuttles appear to be $15 per person, and I'm finding cheaper hotel options. Happy to share any of that info if you are interested.....just let me know....