I have a basic understanding that level change will pitch the a/c to maintain speed with either increased or decreased thrust.
Pipertommy: The basic principle behind LVL change is; for climb mode, A/T gives you climb N1. For descent mode, A/T gives you idle N1.
(However for descent mode, after the thrustlever is closed, A/T goes into arm, which gives you manual control of the thrustlever, and A/P pitch goes into Speed mode. You can then reduce your V/S by changing the thrust)
If you want/need a specific ROD then you normally would choose V/S.