En el que el hosting te permita usar
Pero la diferencia lo dice PHP
Cita: shell_exec
(PHP 4, PHP 5)
shell_exec — Execute command via shell and return the complete output as a string
Cita: system
(PHP 4, PHP 5)
system — Execute an external program and display the output
Cita: exec
(PHP 4, PHP 5)
exec — Execute an external program
Cita: passthru
(PHP 4, PHP 5)
passthru — Execute an external program and display raw output