With the Solar Folio, folding it behind the iPad also disables the keyboard. With the Ultrathin, even taking it off and laying it on a nearby table while you read, watch a movie, etc. isn't 'enough' since the Bluetooth connection is still active. I had stopped reading a book and decided to answer some email and initially wondered why the on screen keyboard was not popping up. Duh, the BT connection was still active.
I think they are both good products (since I do not have the magnet issues others have), but you have to think about how and where you use the iPad before buying.