Start the senior, or it will be a very very rough season for you. Red shirt both QB's if you are able to, then next season creat formations that will give both playing time and get them both starts fulfilling your promise while allowing the STL QB to grow. Your form IQ isnt going to be great some they will struggle, all three while they are playing. However in the later season they should be fine. Schedule easy and pick up as many wins as possible, it will be a tough road with out the IQ especially in that SEC