The problem is that you have to register it to be able to use it. You can call to register it but I think they only activate them at certain points during the day. If you are renting, it may not work for your trip from the airport to the resort. The mini can not be moved from car to car either. If you will be making lots of trips to Orlando then buy the portable (this is what we did).
Also many of the toll roads around Orlando are EPass, not SunPass. While they are interchangable, EPass doesn't provide a discount for using a transponder like SunPass does. So other than for convenience there is really no big savings.