Reasoning is a process of evaluation. Aptitude is capability. For example, if someone has an aptitude for math then they can reason out the processes more easily.
Aptitude is certain capability like thinking deeply, taking things logically, learning technical subjects, teaching, training people etc. It is evident in a person’s dealing, working and general behavior.
Reasoning is an operation done with respect to discussion that is ongoing, a topic that is being written and/or prepared, explaining events, processes, procedures, mechanisms etc.