The main difference is the screen resolution. A3 allows you to output component video. Video is beautiful on this. It's really nice to be able to just drag and drop videos onto it and play them without having to convert first.
No case was included with mine. There is a photo
here of what's included. Mine did come with a short USB host cable for connecting to other devices like card reader or camera to copy files to the A3.
"Portable" means different things to different people. Yes, it's portable in that you can easily carry it with you, especially if you plan to put it in a bag or backpack. It will fit in your pocket, but I don't think you would normally carry it around in a pocket. It's fairly large and heavy for that.