FYI: SLiM (_S_imple _L_og_i_n _M_anager) should allow you to cycle through
the available DM's by pressing the F1 key.  You simply stop on your desired DM,
then login.

SLiM config within /etc/ somewhere ( /etc/slim.conf or /etc/slim/slim.conf
probably) can be used to define the choices.