Unfortunately the true Signature Seats no longer exist. They exited with the last of the Boeing 717's. These were basically first class seats.
Your choice(s) now are a little bit confusing...The only chance of getting a better seat is on the Embraer 190 (the Embraer 170's are squeezed in at 76 seats with 31 inch pitch from row 1 to the back). Currently Midwest is operating different configurations of the 190 but going forward the plane will have 11 first class seats (ie Signature) inherited in the same configuration from US Airways.
The 190's that will be flying for Frontier are suppose to eventually be just regular coach seats with extra legroom. But for now, some 190's flying on F9 routes may also have 11 first class seats.